Hi! I i apply mask to several objects, and later i want to edit/delete them, how can i identify them? i cant see them in the "Layers and Objects" panel. Any way? Thanks!

Answering to myself: there is a little icon with a mask in the things that have been masked. I noted it later. 
Another question is: Is possible to edit the mask without unmask it?
And, Is possible to mask an already masked object?

If you are masking with vector/path, there is a button in the Node Tool controls to see/edit the masks' nodes.

Masking masked objects can have erratic results. You can try grouping the masked object before applying another mask.

Thanks very much Tyler! Will try.