The toll areas have lights on the backsides, which know two things, one if you don't have a transponder at all, and another light for have a transponder but no billable account (i.e. it hasn't been able to charge funds, they of course run a balance for you, don't just bill you for each charge).
The ticket for toll violation runs mid $200's.. so you'll have to factor whether the risk is worth it. On some areas of the Fastrak here in socal (known as the 241 fwy) there is typically a cop stationed in the morning looking for violators, but other areas, like the 91 fwy, is few and far between.
What was funny is a loophole for government workers, a provision developed in California to protect the identities of government/policeman to avoid retaliation or people finding out their information.
They can run through the tolls, but the fastrak people can't pull up their information to send violation notices.. so there are thousands of people who just roll through those bad boys and never get charged.
Loophole lets California workers get out of paying vehicle fines | abc30.com