I normally recommend you look at the front of the box with a flashlight. The laws of physics say that the material in front of the IR receiver must be transparent, although maybe heavily smoked in the visual spectrum.
I assume you have already googled 'where is the IR receiver on the X1' ? I just tried and there are quite a few good suggestions (including using a flashlight )
The internal IR transmitters in the Slingbox M1 are very powerful, and as Sling say, would normally be OK as the IR signal will bounce off walls, cabinets etc so does not necessarily have to exact line of sight.. However, if your X1 and the M1 are more than a couple of feet apart or seperated by a cabinet wall, then they might struggle, hence the provision of the optional IR Cable.