Ask this one early in the process and you could save some time. It's a deal-breaker.
Insurance agents are licensed by the state. Getting the license number speeds up your background check with the state insurance commissioner; it sends a clear message to the agent that you intend to do a background check.
You can laugh it off as being precautionary and being a waste of a phone call if that gets the agent to show you a license, but you should make it clear that you won't hire an agent who can't prove that he or she is currently licensed. Some states give agents a card to show, others a certificate for the wall. It doesn't matter; ask for proof, look at the document, check the names and dates.