Several companies, including Comcast and AT&T, offer online data speed tests. These tools measure how long it takes for a device to download and upload files and calculates the speed based on this result.
Internet speeds can vary significantly, even over a short period of time, so it's often best to run the test multiple times. However, these tests can send several megabytes worth of data at a time, and these transfers count against data caps. It's important to note that mobile data speeds vary based on the type of connection the phone uses. If there is no T-Mobile LTE antenna available in a particular area, a T-Mobile-connected phone might connect to a slower 3G antenna.