This is a nice easy one! Just find a digital camera (or camera phone) and switch it on so that the screen shows a picture (don’t press the shutter button)
Hold the front end of the remote control up to the camera lens so that you can see the small black “window” on the screen. If the remote is working properly, you should see fast, bright flashes of white light on the camera screen when you press a button on the remote. If the flashes are dim, the batteries probably need replacing. If nothing shows, your remote control is likely to need repair or replacement.
It is worth noting that some smart phones have a filter over the camera that blocks infra-red light (one such example is the main camera on the iPhone 6) – in these case, if there is a second smaller camera fitted (eg. the iPhone 6 FaceTime camera), this often doesn’t have the IR filter, so this can usually be used for testing remote controls.