mapping dot to trigger omni completion

Towards the end of `:help ins-completion` there's mention of mapping `.` to trigger omni completion when possible:

inoremap <expr> . MayComplete()
func MayComplete()
if (can complete)
return ".\<C-X>\<C-O>"
endif
return '.'
endfunc

I like this idea, but have no idea what might be used for the "can complete" if-condition. Any ideas out there?

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