How can I tell if my phone is locked? Thanks to a helpful commenter at a previous question, I now know that Starting February 11, 2015, All Four Major US Carriers Will Let You SIM Unlock Your Phone.

But that just raises another question: how do I know if my phone is locked in the first place? Is there some place in the phone's settings menu I can go to determine this, or do I have to get in touch with my carrier and check with their records?
You could try putting another SIM card in your phone and see if it works, if it doesn't, it's locked.