Many servos have a "key" meaning they will only go in one way. The key is the shape of the plug that goes into the receiver. If looking at it from the front of the connector, you will see the shape is flat along one side and has an angle on the edges on the other side. Match this up with your receiver.

Many servos have the negative or black lead on the outside with the signal lead closest to the inside of the receiver. They all go in the same way. And, they all are common to the power source, meaning all the negative are common and all the + leads are common. This means that you can plug a battery into any of the slots as long as the leads are correctly positioned.

The one pictured is a Spektrum AR600 receiver. Note the red circle showing the power and signal lead.

CGr.