Detecting when in omnicomplete

[Karl Guertin]

I'm playing around with keybindings and omnicompletion and I'd like a
special set of keybindings when pumvisible() and it's an omnicomplete
match. I can mostly do this by setting a buffer variable in all my
mappings that would cause me to enter/exit omnicomplete, but I keep
missing edge cases. Is there a better way to detect which completion
mode vim is in?