Most people drill through the top because the top rarely has lines in it for mini fridges. At least the ones that are popular for turning into kegerators.
However, to check for refrigerant lines:
Mix up some corn starch and vodka and "paint" it onto the fridge. If there are refrigerant lines, the heat will cause the vodka to evaporate leaving a line of the corn starch. Probably the easiest way that I can think of.