How to quickly select everything in a layer?

[eobet]

Things I've tried so far:

• Double clicking in various places on the layer in the layers panel.
• Right clicking and looking in the context-pop-up  menu.
• Look in the layer or select menu.

I hope I'm missing something obvious. I have several hundred objects, and multiple layers. I don't with to scroll around in the layers panel all the time.

[v_kyr]

For which tool? 

Usually when you select a layer it's contents should be selected. But there's also "selection from layer" by Cmd-/Ctrl-clicking the layers panel thumbnail image. - Beside that there is Cmd-A/Ctrl-A for selecting all, aka everything in doc.