Make sure the tube hasn't been crushed by someone overtightening the rings.
Bring a flashlight. Look inside the objective lens and down into the scope for any loose particles or paint flaking off inside the tube. Many scopes are coated with a dull black paint inside. Some scopes have lenses that are epoxied in, even quality scopes like March. Shake the scope and listen for a rattle. A rattle is bad.
Look for signs of fogging which might have left moisture marks inside the lens. This would indicate bad sealing which would need repaired.
Make sure the eyepiece doesn't wiggle or wobble, even with the lock ring loosened. Check the turrets, zoom ring and eyepiece for smooth travel.
Make sure you can focus the reticle near and far. Also make sure you can focus the picture near and far. Make sure you can get a clear picture and a clear reticle at the same time. Check the reticle is not tilted off-centre.
Check that the lens coatings are intact and not damaged by caustic chemicals or abrasives. Look for signs of mould. Yes, mould. It can grow inside the glass.
Make sure any important screws are not stripped, like the zero-stop screws. They are small and damage easily.
Take your time and have fun.