ORIGINAL: ameyam
Actually doesnt answer my query. I know that the servos are technically good but, practically, how do you check whether a particular set is good or not.
You "check" them every time you fly the plane!
All kidding aside, the best you'll be able to do is to mount the servo and while exerting a bit of resistance to surface travel, move the servo through it's motion range via the TX.
Look for an unusual "grittiness" or stopping as you do this.
Doing so will not turn up a bad pot or one that is going, but it may uncover a bad gear.
That's about the best you'll be able to do.
ORIGINAL: ameyam
Also, what will happen if one of the servos stops working on a biplane during flight on the ailerons? There are 4 in all, so will the others help compensate?
Ameyam
It depends upon the failure.
If the servo stops fully deflected, you may not be able to control the plane. If it stops operating in a neutral position, you may not have any problems.
A servo can always fail in use, so that's the risk you take every time you fly.
Strangely I too have had the MOST failures occur with Hitec's over any other brands INCLUDING the cheap Chinese knock offs.