Or you could try connecting a battery to the cable at the amp end, and hooking up the speakers at the speaker end. Connect the amp end - terminal to the - terminal of the battery and hook up the speakers, then with someone watching at the speaker end, touch the + cable at the amp end onto the + terminal on the battery.
If the speaker bass cone moves outward, you have the cable connected correctly to the speaker. If the cone moves inward, you have the speaker connected in reverse.
So now you know which + and - conductor at the amp end corresponds to which + and - at the speaker end, and problem is solved.
Just about any battery will do, but I've often done this with something like a 9V PP3/6LR61/1604A as this gives a very obvious speaker driver movement, which you may not get with a more common 1.5V battery.