Is your seat belt done up?

Both are fine. The first is more informal, likely to be said by a parent to a child. The second said to an adult.

Thanks, Dave!

What about imperative? Are these fine?

Please wear your seat belt.

Please fasten your seat belt.

Please do up your seat belt.

Thanks again,

Tom

All fine, as before depending who you are addressing depends which one you would use.

Thanks!

I understood "do up", but I'm yet to know the difference between "wear" and "fasten".

Tom

Wear you seat belt means to have on, the same way that you would wear anything. Fasten your seat belt could infer that the person was holding their belt in place rather than having it secured properly.

I might say wear your seat belt if I thought the person had no intention for putting it on and fasten your seat belt if they are likely to but just haven't yet.