Have you looked at other sides of the SMPS box? Even if you could open the box, it wouldn't tell you anything about its power rating unless it's written on the inside, which is highly unlikely.
Anyway, hard disks don't use a lot of power (around 10W) and fans use even less (~3W). So, unless your SMPS was just marginally enough to start with, adding a HDD and a fan should not be a problem.
If you described your configuration, it might give us an idea of what the electrical load is like at present.