The condo association should be able to provide you a history of all past special assessments.
It should also be able to provide you a list of any upcoming (but not yet collected) special assessments.
Even if your question is about upcoming possible special assessments, take a look at the items mentioned above. You may see a pattern. For example, some condo associations tend to keep their fees low by loading long-term maintenance into special assessments. For instance, roof repair is something that ought to be budgeted for, and generally included in the regular condo fees. But some associations handle roof repairs on an ad hoc basis, assessing and collecting on an as needed basis. So, if you see periodic special assessments for roof repairs, you might reasonably assume that they'll do it again.
Another tip: Talk to current owners there. Ask what their experience has been. Sure, they'll tell you about past special assessments. But they'll probably also know about possible upcoming ones, or major issues or needs that would require a special assessment once addressed. I lived in a condo once with an underground parking garage with a leaky roof. We knew for a few years that it'd take a special assessment to raise the money to fix it. So even though nothing had been voted on yet, if you'd asked me (or the other residents), we could have told you about it.
Hope that helps.