What am I looking for?

I have a couple of times and couldn't see a thing, I could only verify
by feel and tugging on the flaperons. I often see owners with
inspection mirrors peeking inside. If I remember correctly, the
connection is somewhere behind the spar, is there a trick to seeing it?

No trick, seriously:

You just reach in and hook them up by feel - one hand on the
connector, and other hand on the control (stick or dive brake) to move
to the right position to snap in place. Verify the ball is in the
socket, then screw the safety all the way on. THEN: Look through the
opening and you can see all 4 connections, directly and on the mirrors
installed by LS behind them. Sleeves are color coded to show if they
are not correctly hooked up.

No additional inspection mirrors needed, since they are already
installed in the ship!

I really fail to see how it could be any easier - except of course,
automatic hookups a la LS6-c...

Beyond that, the fact that I know two very experienced long term LS6
owners who managed to screw up that connection is the main reason I shy
away from them...

In the above sentence, replace LS6 with just about any glider,
including those with automatic hookups, and it will still be true.
Anything mechanical can fail or be mishandled...

Anyway, I hope you like your ship, cuz I love mine!

Kirk