Only way to tell really is to take along a compression tester.

See if the seller has paperwork to prove recent rebuild.

Check head nuts for rust - if they're bad the head hasn't been off in a long time.

Check the powervalve works by rotating the motor when the ignition is off. It should be stiff but move - if it doesn't it's well gummed up and I'd walk away, as this can burn out the motor and blow the CDI. Not cheap.

Pull a spark plug and check the colour - white is lean and about to / already has detonated.

The bike should start in 3 or 4 kicks. If it doesn't it may be a bad mixture, but it may be crap compression.

Ask what mods have been done - the 18 was unrestricted until the very last few.