QF1Y: Mechanics Question Lead/Trail

[ca_rumperee]

Lets see how well I can explain this relatively common situation...

2 man mechanics, trail is table-side, A1 has ball, pretty much right in front of trail. Entry pass into post player A2 who is on the block. Lead is on the far side of the court, lets say with other players moving through the key (semi-obstructed view).

So, entry pass is:
a) errantly thrown, just goes out-of-bounds on endline.
b) is muffed by A2 out-of-bounds
c) is deflected by a reaching B2 defender out-of-bounds
etc, etc.

What is a rock solid way to handle this out-of-bounds violation call, from both the lead (who, I'm suggesting, has a poor angle to see who last touched this ball which has gone out-of-bounds on their line) and trail.

As the lead I have just blown whistle, looked to partner and gotten no indication (like, "what do you need me for?") and as trail I have blown the whistle and felt like I had overstepped my bounds by calling (or trying to) on my partners line.

This seems like a fairly common scenario, so looking for some advice on how to really do it right.

Word.