First of all, get the boat surveyed.
(see the post: Chris Craft 28 Catalina)
Look at the engine compartment, if it's clean and looks in order you might think it was taken care of.
Check the fiberglass for cracks and or bubbles.
Check the lights, blowers, bilge pump.
Check the cabin and bilge for signs of mold or mildew.
Take the boat for a ride: Check the oil press.. How does it run? Does it tend to pull to one side more then the other? Check the trim tabs & tilt cylinders if boat has them.
Check the engine oil, is it clean? Does it smell like gas?
Is it thick or mixed with water?
What's the over all condition of the boat, does it look like it's been taken care of.
Ask the owner if it's ever been beached or hit anything (sandbar, etc...)
Ask to see maintenance and repair records.
This is just some of the guidelines I went by when I bought my boat 5 years ago.
Others might have suggestions I didn't think of.
Good Luck,
Ggg