Validate field that there is a comma

I want to have the user enter their name as Last Name, First Name, with the comma. I need to be able to validate that there is a comma, figuring if there is not, they did not enter the name correctly and will give them a message saying such.

I have found a few things, but they don't seem to be working. Plus not sure where it would go, exit, change, validate, etc.?

I'm thinking something along the line of an e-mail validation, where it would check if there is text before and after the comma.

Thanks, Gary