[somedudethere]

I'm sure in the context of the dinner party you would know ("hey did you try the beaf, hard to tell its vegetarian", or its a vegetarian dinner party, or usually at dinner parties there is ample notification for allergens if its catered or if it is home prepared, the host usually proudly tells everyone what he made).

Or in the context of giving an order you would know. No one is trying to pull a fast one on you. Granted non-english speakers would probably have a harder time figuring it out. Its a problem but not as big of one as you make out.

Rarely do you go into something completely blind and oblivious except for the punctuation of one word.

[Zancarius]

In my own personal experience with food sensitivities, it'll be the last thing that crosses their minds. It's not necessarily that the hosts in this case are intending to "pull a fast one" on a potentially allergic guest as much as they simply don't think of it. Most people don't have food allergies, and unless you mention your own issues, it's not likely to come up in conversation because it's the absolute last thing on their minds. This is especially true for rare and very uncommon allergies and sensitivities.

Now, granted, if you have such an allergy, it's important to make sure anyone preparing food for you is made aware--but in the context of this hypothetical dinner party, if someone is introducing a meat substitute, there's always the possibility that they're going to remain silent in order to gauge their guests' reactions. That's where the real danger is: Guest assumes meat is, in fact, meat; host wishes to see guest's reaction to meat substitute. In such circumstances, the guest isn't going to ask if it's made of a legume because he or she has already made the assumption that it's meat.

Alas, this illustrates that food sensitivities require those of us who suffer from them to remain vigilant.