It all depends on what the wire is connected to at the other end. Typically on a wire like that the white stripe would indicate the positive connection but wire is wire, you can hook it up to either terminal as long as the connection on the other end matches. The only thing you want to avoid is hooking negative to positive and vice versa. Can you show us a pic of what the other end of the wire is connected to or at least give us a description of what you're seeing?
There's no way to know and it doesn't matter! Sure, if you hook it up "wrong" the initial movement of the speaker cone will be away from you, but even at the lowest E on your bass guitar that's only 1/80th of a second delay, which you can't see or hear (12 msec). The only problem comes when you wire in a second speaker, you want the cable to be wired the same way.
So, here's what you do… You get an ohmmeter and check which wire the tip is connected to. Then, any further speakers or wires you use, you wire the same way. By convention, the tip is usually designated "+" and wired to the white stripe wire.
OK, lots of knowledgeable but inarticulate responses here, guys.
To tell which strand of the wire is positive, set your ohm meter to check continuity, usually with an audible beep to indicate a closed circuit. Attach one end of the meter to the tip (end) of the 1/4" male plug at the other end of the cable. Attach the other end to one strand and then the other until it beeps. The one that beeps is positive.
To check which terminal on the speaker is positive, use a 9 volt battery and a couple alligator clip leads from both poles on the battery to both terminals on the speaker. When you make connection, the speaker will emit a click and move either forward or backward in its frame. when you orient the leads where the speaker travels forward, whichever speaker terminal is attached to the battery's positive pole is the positive lead.