The Legion of Super-Heroes is an exclusive club, complete with a constitution, hazing, and a club house. And its own secret handshake, of course. Here it is from Adventure Comics #290:
This is him essentially demonstrating the handshake to a fake Sun Boy who (literally) showed his hand by not using it at the appropriate time. Problem is, you can't go showing this stuff to an infiltrator if it's what you use to defeat infiltrators.
Oh well, it was kind of simple anyway.
Back to the drawing board...