Foam, bubbles, all that stuff can pretty much be ignored. It seems like the telltale is if there's a "skin" covering the top surface.
Fill with mingled cream and amber,
I will drain that glass again.
Such hilarious visions clamber
Through my chambers of my brain.
Quaintest thoughts - queerest fancies
Come to life and fade away;
What care I how time advances?
I am drinking ale today.
- Edgar Allan Poe