Even with absolutely no foam at least gravity can't lie right?
The foam can come and go in the course of a few hours if your fermentation is vigorous enough. As others said a slow ferment may not produce a Krausen.. But I'd be concerned about stress yeast flavors in that case.
Fermenting: Citrus IPA
Drinking:American Wheat, Pale Ale, Slovenian Blonde, Milk Stout
If I could figure out a way to SCUBA and drink Hombrew at the same time...well...I guess there wouldn't be anything left to try to attain in this life.