Here's how I check if a remote is functional:
Make sure the batteries are working. Usually, I pull out batteries from another RC that is functioning properly. This is to eliminate the possibility of battery malfunction. Install the batteries on the RC. Then, grab your phone and open the camera app. Point your RC at the camera and press any button while looking at it on your phone's screen. Check to see if the LED bulb on top of the RC emits a faint light flash every time you press a button. If it does, then the remote is sending an I/R signal and should be working properly. If there's no flashing light, then the RC needs servicing.
Hope this helps.