Some general tips I usually try to think of are:
* If the value in question is more than 6, for example cos(15), you're more than likely in degrees.
* If the value has π anywhere in it - cos(π/6), cos(π/2), etc. - you're in radians.
Generally, radians is the default the higher up in math you go, and I would recommend trying to think in them by default unless told otherwise. If you could give us the specific question, we might be able to help you see if there was maybe a context clue you missed.
Lastly, as this is an exam question - in an actual exam, I think there would be no harm in asking the proctor which one to use.