If you have not yet cut off the connectors, the center conductor should be the "+" terminal for the speaker connections.
You can cut off the connectors and then connect the individual wires to your receiver. Switch the wires until you get sound from both speakers. This will not hurt the speakers or the receiver if these are indeed the speaker wires. Alternatively, you can use a continuity tester to identify the speaker wires.
If you want to go one step further and make sure that both speakers are connected "in phase" (so the +/- terminals are connected correctly), then try this test. You will need another person and a AAA battery. Remove the wires from one speaker from the back of the receiver. Have the other person stand on a chair and look at the speaker cone while you connect the AAA battery to the two speaker wires. The speaker cone should move either "out" or "in" when you touch the wires to the battery, and then the cone will return to its starting position. The wire that makes the cone move "out" when it touches the "+" terminal of the battery should be marked with tape. Do the same thing with the other speaker and its wires. The speaker wires that are marked with tape should then be connected to the "+" or red speaker terminals on the back of your receiver.