My engine burn a lot of oil (2-3 L for 500 km)

I have opened it and i'm looking for the problem...

The 2 heads seem OK (I still have to check valve seals)
The cylinder seem perfect (no stripes) but how could i check if I have the famous nikasil problem ?
I can give photo of the cylinder if you want !
But i have not yet dismounted the pistons.

thanks