just use common sense and listen to your gut. This is very basic tire kicking stuff but will give you enough to move on or not with out posting a very long thread.
Is wiring neat and tidy, any plumbing should also look neat and tidy, take a flash light and rag !!!
pull all hatches, doors, ... go over every inch( show the owner the light and rag and tell the owner your gonna need time and really look at the boat, his/her answer here will be really telling) and then open her up ,really look in there with the light.
Ask the owner what they added on... then ask how they did it, simple slapped it in vs. multi step process the correct way? How is it stored? full of leaves, mud or in decent covered shape.
If these basics seem OK, then go for more thorough.