This is a pain point for me as well since my son is allergic to eggs and tree nuts. I have occasionally found flights where on board they had some allergen info. You could always ask the FA if they have such a list.

There’s a thread in this forum where people post pictures of their food and sometimes commentary. It’s a lot to wade through but it’s something you could look through if you have time.

I suggest flying Air France if you ever travel from EU to USA - they have complete allergen lists onboard and also if you click through the right things on their website (I think it’s buried under flight status) you can see the meals and their allergens a few days in advance. AF did not have allergen lists ex-USA either.

Of course on Delta there is the allergen-free special meal...