How do you normally respond if you don’t know the answer to a customer’s question?

You can always tell the customer that you will check and get back to him with the proper answer

This is the worst moment for an employee to lack answers to respond to a promising customer , The best course of action you  request the customer to wait for a minute and you call for help from your fellow workmate who has the best experience and has clear information about the customers question this where team work is been played among the the employees . The customer will get the revelated information he or she needed to know .As for you need to take the step to learn more from your fellow workmates so that you can be the masterpiece next time to a new employee or  prospective customers . 

First admit about unawareness to built trust.

Secondly, assure of coming back with a timelined appropriate answer, to make sure no call back by the customer.

Thirdly go back promptly on time, to keep up the commitment.