Don't know which frequency I'm using

I am a first time iPhone user, switching from an old Android. How do I find out which frequency (CDMA/GSM) my iPhone 5c is utilizing? Is it anywhere in settings? I purchased the phone through Straight Talk (don't judge, please) and they are so secretive about this. I don't know exactly which carrier towers this phone uses. I had assumed it was using CDMA through Verizon, but the phone has a sim card installed and I get LTE displayed most of the time. Some people tell me that ST uses at&t, which would mean GSM. So, how do I know for sure? If you say check the model number, that won't tell me, because the same model number is for CDMA and GSM. Could I take it to an Apple store and have them research it, or is there something I can do myself? I've tried the Field Test, but it's pretty difficult to decipher. Any suggestions?

Well, after following your instructions, here is what happened. When I placed a phone call (wifi disabled) I saw it change from LTE to "3G" next to the carrier name. Then I called again (wifi still disabled) and opened up Safari and began browsing, and again it showed "3G". So that tells me that my phone is a GSM phone. Thanks for the explanation. Now I know what type of frequency my phone uses. Thanks again!