Refrigerant lines are normally visible on unit's like this. There should be a condenser coil mounted to the outside back of the unit, or underneath the unit, with the compressor. The condenser coil is the one that get's warm or hot when the unit is running.
As for the evaporator, or cooling coil, it is what get's cold when the unit is running, and is normally mounted along the top or back of the unit. As long as you avoid puncturing any of these two coils, you should have no problem drilling holes. However, if this is a conversion kit, there should be instructions for where to drill the holes. If you just bought a refrigerator with the intent of converting, well, you'll need to look the unit over real well using the directions I've given above. The only other thing to look our for is the suction line and liquid lines leading between the inside and outside of the unit. You should be able to find where the enter into the cold box on the inside, and where the exit the cold box on the outside, near the compressor/condenser coil. Manufacturers normally use the shortest route between the two to save material, so you should not have to worry about anything in the top surface. One way to tell is if that surface heats up when the unit is first turned on from not running.