Well it's relevant to buying a used 2 stroke so I'll explain it in the thread for the benefit of others ...
The "rap" test has to be done with the head and cylinder off and the piston off the rod ...
With the crank at TDC and locked there ... you hold the rod near the bottom , push it down onto the crankpin , then rap the rod small end with a block of wood or the wooden handle of a hammer ...
Take note of the sound you hear ... it'll sound solid ... Next pull the rod upwards then repeat the rap on the small end ... IF the sound you heard changes as in a click , knock , or anything different ... then the big end bearing is worn creating excessive up and down rod play ... so a crank rebuild is necessary ...
If you don't fix the crank ... and fit a new piston and rings ... the play will pound the big end roller cage to an early death and that in turn can take out the cases , cylinder , piston , rings , head etc ...
The only other way is to know what sounds a freshly rebuilt engine of the same model makes ... then listen for any extra "rattles" or noises in the engine of the bike you're going to buy ...