Don't get me wrong--I think it's a worthy point that reading labels shouldn't be part of Orthodox fasting. Fr. Pat is technically correct when he says that the Ninevites and Jesus didn't read labels. I'm just not sure things are ever quite so simple.
For starters, these are great examples of fasting, and if we were to follow them literally, our fasting would look quite different. We probably wouldn't eat anything, which does take away a lot of the guesswork. On the other hand, we might struggle a bit with how to align the Ninevites' very public fast with Jesus's instructions to make sure that our fasting isn't obvious. Perhaps the different contexts help--the Ninevites were fasting as a community, so there was no point hiding it. When fasting as a matter of individual ascetic practice, it's better to keep it a secret. But that gets us to what I think is the real issue here--how do we fast in the here and now?
Orthodox may all fast as a community (that puts us somewhat more on the side of the Ninevites), but particularly here in the West we do so as a minority community. Our friends and neighbors often aren't fasting with us. In some cases, even family members aren't fasting together. Indeed, within a given parish, some may fast very strictly, while some do not fast at all. So the individual dimension is pretty hard to get around.
Now, for the specific issue of reading labels. If we start from the high standard of Jesus and the Ninevites, it's very easy--just don't eat anything, and everything's safe (or nothing is, but the outcome is the same). But generally speaking this kind of observance of Lent is discouraged as too extreme. And arguably, even if one is capable of fasting so strictly, one shouldn't. It seems to me that it would be very difficult to do so and avoid the sin of vainglory.
There are only a few days during Lent in which it is even considered that one should fast with this degree of strictness (Holy Friday and Clean Monday, for instance). The next step down is bread and water, which could also be done in a pretty straightforward manner, without reading labels. That is, it could be done if one first knew to stick to basic yeast breads, where dairy products are less likely. Of course, making the bread yourself is always a good way to know for sure.
Beyond that, we get into things like soups and salads and various other concoctions of vegetables and legumes. Again, if you prepare them yourself, it's generally quite simple to avoid meat and dairy. If you buy them pre-mixed in the store (to say nothing of prepared foods in restaurants, etc.), it can be surprising what's in there. And this is precisely the point. If you go back just a couple of generations, you get beyond our current obsession with pre-processed foods. Whatever minimal processing was done was typically done in the home. Things didn't come already mixed together. You might buy a chicken from the butcher already killed, plucked, and ready to cook--but it was just a chicken. Anything else that went in the pot was your business. These days, it seems the exception rather than the rule for people to prepare their own food truly from scratch. It's because we buy things that come with labels that we have to read labels.
Another issue here is the learning curve that converts face. Presumably generation upon generation of Orthodox housewives learned from their mothers and grandmothers what recipes to prepare during Lent. You didn't have to think about it--you just knew the right dishes. Not to mention, much of this tradition had a chance to develop where meat was truly a luxury. Cooking vegetarian was a way of life and an economic necessity, even aside from fasting. These days, many American converts have for the first time even to think about cooking and eating without meat or dairy. Eventually you do figure out what's safe and what's not. But getting to that point takes some study, at least part of which normally involves reading labels.
Of course, we could avoid a lot of this hassle if we just stuck with the basics--homemade breads, soups, and plain veggies. For many people, it would mean spending more time on food preparation during Lent than otherwise, which may be counterproductive; but if you do it with the right attitude, I suppose it could be a spiritual exercise to make things from scratch. If you have little kids, you might need to be a bit more creative. Given a choice between soup and starvation, I suppose at some point they'd pick the soup, but few parents have enough patience for that kind of stand-off. So, you start looking at more prepared foods, snack foods, desserts, etc. Would it be better just to say that they must be too young to fast? Or do they learn at least something about fasting by restricting themselves to Oreos for two months?
And then there's the complication of social interaction with non-Orthodox adults (some of whom may live in the same house with Orthodox). Do you refuse all invitations because it's Lent, or do you insist on picking a vegan restaurant, or do you call love an excuse for breaking the fast every time? I don't think there's a one-size-fits-all solution here, but at least some of the time you can just quietly pick things that don't break the letter of the fast (and if you're genuinely participating to avoid offense or out of love, you're probably not breaking the spirit either). If you've spent enough time reading labels, it actually cuts both ways. Sometimes it's easier not knowing what's in everything so you can plead ignorance, but sometimes you find pleasant surprises, where something you wouldn't have thought is OK really is. NOTE: I am not advocating reading labels while you're at a party or someone's house. My point is that it helps in such cases to have done some homework on the most common packaged foods.
I'm not saying that reading labels has to be for everyone. Personally, I'd rather not do it, because I know I can get too legalistic about such things. If you can get by with homemade bread and soup the whole time, God bless you! You're better for it. But in some situations, I think there's good cause. The trick is moving beyond label-reading. At some point you hopefully settle down into enough of a routine that you don't have to think about it anymore. You figure out what works for you, you know how to adjust when interacting with others, and you keep your mouth shut when someone does something differently. Reminders from the pulpit about not reading labels are good for pushing us in the right direction and instilling the right attitude; I just hope they understand the practical challenges involved.