Some hard & soft bits.
In the strange but true category -
As far as safety this is #1 for me: (not so much for a purchase decision but for safety before driving it.)
replace the steering hub to steering shaft bolt or at least check it. Mount the bolt so gravity causes it to fall into & not out of the hole. (Ask me how I know)
All Safety wire, Safety clip or double nuts should be installed before driving.
Not the kart, but look at the driver. If he is of a vastly different build you probably won’t be dead on as far as setup & fitting. Probably not even if it was your twin. You can also get a general idea from the environment in which the kart is kept.
Smashed or flattened frame rails where engine mounts
rear Bumper & mounts, including chassis tubes & bolts/rubber going into chassis
Seat Mounting tabs & stays
Side pod mountings
Correct spindles for the kart (again, AMHIK)
I’d also throw some snipers on the front if at all possible.
Bolted on items
Not sure if you consider rear axle & bearings as chassis, but I’d make sure the 3rd bearing is in there & included along with bearings being usable. Straight axle and look @ the stiffness. If it is at on end of the spectrum you probably need a different one.
In general make sure all parts are included, like front & rear bar, side pods
Seat condition & size, it is a bonus if you can use it.
Superfluous BS: Don’t get blinded by a bunch of junk included that you realistically can’t use.