Bubbly monster" is the best way to put it. That's when you know it's really alive. I had to start feeding mine twice a day when it got that active. The smell changes too, gets more vinegary and less like flour.
Had a jar crack once from the pressure. That vinegar smell @the_anthony mentioned is a solid sign, but the real test is how fast it bounces back after you stir it down. If it foams right back up in minutes, you've got a beast. Might need a bigger jar or a looser lid before it makes a real mess.