How to test power output of an 8.5kW generator?
Borrow some electric kettles from your friends, or buy a couple, they're not that expensive.

They're compact, and light when empty. 

They have a very predictable current draw, not something that could be said for welders or angle grinders for instance. The power factor is unity, and there's no inrush current, so there will be no argument with the vendor that you're testing the generator under ideal loading conditions.

You need only three 3 kW kettles to get to up to 9 kW. See if you can get one or two lower power ones as well, so you can adjust your load finely to see what collapses the generator output.