Request copies of the strips from any photo booth company.(If they cannot email them to you this is a red flag)
Request a copy of their insurance.
(many venues do not allow uninsured photo booths into the facility)
Request a proper contract and never pay in cash.
(Make sure the contract covers you and the photo booth)
Ask about the booth – what is the manufacturer name.
(If it is not given this means it home made or made by themselves)
Ask what printer is installed inside the booth.
(If they tell you it sits on a table that is another red flag)
Ask what type of camera if installed in the booth.
(Some may not be able to tell you, but if they use the word webcam – run!)