Very wide open question you're asking. I once sold a boat to a gentleman and he never put it in the water. Lucky for him, I'm an honest person - I would never have done the same.
1) Review the hull for dents, scrapes, blisters, holes...etc.
2) Put the Cruiser in the water - ensure it floats.
3) Check all electrical - ensure all breakers work on the panel and all electrical/electronics work.
4) Lift all hatches and review for rotting.
5) Run engines. Evaluate engine hours. Ask when engine was last serviced. Ask if they'll tell you who services the engines.
6) Run the Cruiser out for one hour or so and check stability, steering, stabs, hydraulic levels, etc.
7) Check battery switch operation.
8) Check batteries.
9) Check battery charger, ionic galvanizer for functionality.
10) Ensure all navigation lighting is operational.
11) Check anchor storage in v-bert for rottting and general condition.
12) Check to ensure the stove works.
13) Run the water lines - ensure the water pump works.
14) Open/Close all windows - check screening.
15) Check that the head shower is functional.
16) Check that the marine toilet works. Boat have a macerator?