On speaker wire there is usually a way to tell one conductor from the other. Sometimes they are marked + or -. Sometimes there is text on only one conductor. Sometimes there is a ridge on one. Sometimes one is copper the other silver.
If you can't find anything then mark one conductor, follow it to the other end, and mark it there too. Usually the marked conductor goes to the plus or red terminal but it doesn't matter as much as getting all the speakers connected in phase. That means that the plus and minus are connected the same for all the speakers at both ends of each wire.