If you have krausen, there's no denying your beer is fermenting. If you've ever fermented in glass, it's quite obvious when fermentation is taking place: krausen forms, bubbles form, and the whole volume roils and boils with activity. In plastic buckets it's harder to see, but the krausen is a dead giveaway.
You don't need a gravity reading to know if fermentation is happening, but you do need a gravity reading to make certain that fermentation has
ended.
If you'd like to see this beautiful sight, but you don't want to ferment in glass carboys (which is completely understandable), make a small beer, as if you were making a yeast starter: Boil a pint (1/2 quart) of water and stir in 1/2 cup of DME. When cooled, pour into glass quart milk bottle, or any glass or transparent plastic vessel. Add a fifth a packet of Nottingham, Cooper's or other inexpensive dry yeast. You can even use bread yeast if you want, you're not going to drink this. Shake it up well to aerate, and come back in a few hours. It's really an amazing thing to see!
This short video is a batch of Ale I caught right at the beginning of fermentation, before the high krausen build up: Beautiful :rockin:
Brew on Brothers!