The only way to determine which wires are the line and which the load is to disconnect one of the hot wires. If that disconnected one is hot with the breaker on, then it is the line. The receptacle will then be unpowered. If the other is the hot, then the receptacle will still be powered and the disconnected wire will not be hot. The neutral paired with a given hot will be ascertainable as being in the same cable sheath.
The back wiring connections in the holes can be of two types. The old cheap and inferior receptacles made a spring loaded connection when the conductor was pushed into the back hole. The side screw had no effect on this connection. This is considered an inferior connection and should not be used even for copper wires. It was prohibited for aluminum conductors. I hope you have copper.
The other type of back wiring (which I have heard about but not seen myself) uses the side screws to clamp the wires in the back wiring holes. Duplex receptacles with this feature are more expensive (~>$5.00) and are considered desirable. I believe in this type one can attach wires by looping around the screw in addition to inserting into the back holes. If I understand correctly, this type allows more branching without the use of wire nutted pigtails, but one of the experts on this site should confirm this.
If feeding an exterior receptacle is allowed by code, you could use a GFCI receptacle in the exterior box.