If you want to be precise, have a friend hold a spring scale. Tie a loop in the end of the line, hook it to the scale, and back off about 30'. With the drag screwed down tight, bend the rod like you're fighting a fish. Adjust the drag so that it starts slipping when you've got a typical "fish fighting" bend in the rod.
The rule of thumb I've always used is set it a 20-30% of the breaking strain of the line. So, if you're using 6# line, you'd set the drag to slip at around 2#. For 2# line, you'd have to set it pretty light.
What line are you using?