Remember that on aluminum electrolytics, the polarization mark indicates the negative terminal, but on tantalums the mark indicates the positive one. As others have indicated, if this is on the -5V rail, then the + side should be to ground. Nevertheless, you should easily be able to tell which terminal is more positive with your trusty voltmeter. If you measure the voltage across the cap as +5V and the positive lead is connected to the + side of the cap, then it's installed the right way.