This is a great question - it comes up often.
If you are using an MTN SIM, it will be branded with the LTE symbol, which is a really quick way to identify if it is LTE capable. You also need to request that the SIM be provisioned for LTE in order to be able to access LTE service. This usually takes a few hours (72 hours abslute maximum).
You need at least a 128k SIM to receive LTE, so if your SIM does not have the logo, you can check if it is compatible that way :)