Hi Tom and welcome to Clockify forum!
There is no option to audit tags. Would you mind clarifying a bit what would you want to achieve by this and how would you use it?
The only alert you can get on tasks is if you set an estimated number of hours for completion of this task and you can set to get an email alert when you reach 50, 75 or 100 percent of this estimate. You can see more about alerts here.
Otherwise, there is no option to receive a reminder to start tracking time on a certain task.
There is no automatic billing in Clockify. You can only mark some time entries as billed/invoiced after you charged for that time, with a “billed/invoice” tag you can create.