How can I tell if a ceiling box will support a real light fixture?
All ceiling boxes are required to support at least 50 pounds (NEC 314.27(A)), so you can assume the box will be able to support a simple light fixture.  Boxes that are designed to support ceiling fans must be labeled as such, and must have a label listing the weight it can support if over 35 pounds (NEC 314.27(D)).
If you look at the box you have, you'll notice a flat bit within the box.

This is designed to sit against a framing member, so the box is likely attached to the member with nails.  Your box probably looks something like this...

As others have mentioned, the holes in the back of the box can be used to attach the box to a support.  In your box you'll notice these holes are empty, which means the box is not attached to a bracket.


tl;dr
This box should have no trouble supporting a normal light fixture, up to about 50 pounds. It should not be used to support a ceiling fan.