[ElDiabloConCaca]

Not in a general sense, no.

In a broader sense, it depends on what you mean by "see what they're doing." Do you wish to actually "see" what they're doing, like where the mouse pointer is, what they're typing, and what is on their screen? Without Remote Desktop (or some sort of VNC process/server), no, you cannot do that.

If ssh is enabled on the machine, though, you can simply ssh into the computer and identify the current processes running under the suspected username. That'll tell you what programs they're using, but won't tell you exactly what they're doing with those programs.

All in all, I think the answer is "maybe" depending on what information, specifically, you would like to gather. Please elaborate on exactly what you're expecting (or would like to "see") and we can help you determine whether it's possible or not.