take a regular 9 volt battery along with you. when you find the speakers that you want, touch the positive and negative of the battery to the positive and negative on the speaker ( you may need some jumper wires for this )
it doesn't really matter if you have the positive on the battery on the positive on the speaker. if it is hooked up pos to pos and neg to neg, the speaker will move one way. hooked up pos to neg and neg to pos, the speaker will move the other way.
it would be a good idea to check both ways, to get the full movement of the speaker to make sure there is nothing bad in the speaker.
this can be done with any battery ( I would not go any larger then a 9 volt ), but I wouldn't really suggest holding it on the speaker for any long amount of time.
hope this works for you