Asking something as simple as, "are you allergic to any food or animals?" and telling them
what pets and food you have should be okay. If your guests know about the party,
they'd likely understand and be more more than willing to tell you if they're allergic to anything.

Preference-wise, you could ask your guests something like: 
"Do you have any preferences for food? I want to make sure that everyone has something they like at the party."
This lets them know why you're asking them what their food preferences are.

Explaining *why* you want to know these things makes it feel less like science research on the receiving end.
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