Welcome Alberto,
You have to look for the same things which you would look for all the bikes.
If the bike is heavily dropped, you can see the exhausts bent to the inner side. The rear fork could be dented by the nuts of the exhausts.
Besides, when the bike is dropped, often the headlight frame goes bent, so the headlight is not centred in the front fairing.
However, if the bike is yellow, for sure it has been repainted because the Paso 906 was only built in red/white, blue/white and grey/white.
All in all, the Pasos are sturdy and have only minor issues from the electrics and the fueling.
So, probably the worst problem is the availability of the tires.
They are built no more by Michelin (the only one producer) since 2004.
So, that will be an huge problem.
Good luck
Ciao