G'day Frechy,
The sump check valve is inside the sump orifice, it's a ball inside the scavenge pipe sticking down which goes through the hole in the cover screen,so you need to get on hands and knees to diddle with it or get a mirror(I have a couple of old side mirrors that are invaluable or buy a telescopic one on a stick).
There's not much you can do except poke it to ensure the ball isn't sticking, other than take the oil pump off and blow air back through the pickup pipe while holding the ball open,hopefully you won't need to do that, though you could poke something in to hold it open fro a while and allow any crud to drain out?
It's all been discussed here recently if you search wet sumping.
It's also been pointed out that if you have a magnetic plug in the cover, it can hold the ball closed, so needs to be orientated as far from the pick-up as possible.
The 'anti-wet-sump' valve is behind the oil pump and can only be accessed from inside the crank case, meaning full strip down, in my opinion unnecessary unless it's real bad.
I hope that's helpful.
Cheers, duTch