You'll know just put your nose down there and take a deep whiff. You need to get an idea of the smells that are normal and even those that are 'off' odors. Hopefully you'll never get those off odors but knowing 'normal' helps you know how things are progressing. (And you can get hooked on those yeasty fermentation smells too. - My wife doesn't care for the smell but I could just sit in a room with those smells all day long.)
Anyway you will likely see some mild to heavy foaming within the first 24-48 hours. IF the temps are at the low end it can take up to 2 days, or longer for fermentation evidence to be seen.