There are three reliable ways to find out whether your phone uses GSM or CDMA as its communications protocol. You can check your phone for the presence of a SIM card, check the phone's specifications provided by the manufacturer, or contact your service provider to find out what protocol it supports.
GSM and CDMA are two similar, but incompatible, ways that phones connect to a wireless network. You need to know which protocol your phone uses because it affects your ability to change service providers and use your phone when traveling to a different country. GSM phones use SIM cards provided by the service provider. Switching providers is as simple as switching out SIM cards. CDMA providers require the phone to include a built-in chip. This means that you need permission from your carrier to switch networks, since not all CDMA phones work with all CDMA networks.
Some phones support both GSM and CDMA. In this case, the phone works with any GSM carrier, but will not necessarily work with any CDMA carrier.
Inspect Your Hardware
The most obvious difference between a phone that uses GSM for communication and one that uses CDMA is that GSM phones have SIM cards while most CDMA phones do not.
Some Sprint and Verizon phones use a SIM for their 4G LTE networks. These phones are still CDMA and work only with those carriers.
On many GSM phones, there is a slot on the side that holds the SIM card. This allows the SIM to be easily swapped. On other phones, you must remove the back cover and perhaps the battery to access the SIM slot. The SIM card on contemporary phones is usually slightly smaller than the memory card and is almost always white.
Refer to Manufacturer's Documentation
On many phones, in particular iPhones, the back cover cannot be easily removed. In this case, you can find whether your phone uses GSM or CDMA by reading the phone's specifications provided by the manufacturer. The specifications were included with the phone when it was purchased, but you can usually find them on the manufacturer's website as well.
In order to find the correct specs, you need to know the model of your phone. This can be found written in small print on the back of the phone, usually near the top or bottom of the device.
Contact Your Service Provider
Almost all service providers exclusively support one protocol or the other. If you cannot identify the phone by the presence of a SIM card or through the manufacturer's documentation, contact your service provider directly, and ask whether the network uses GSM or CDMA.
In the United States, the major CDMA carriers are Verizon and Sprint. The major GSM carriers are AT&T and T-Mobile. There are many smaller GSM carriers, as well.