As the others said, 120 film doesn't have a leader per se. But the roll is long enough to leave a little extra for you at both the beginning and end of the roll, from which you can snip a small strip without intruding into an image. I recall that with the Holga (like maybe all MF cameras?) the extra margin is something like 1-1/2", or maybe a little more. This extra exists at both ends, so it doesn't matter from which end you start rolling onto the spool or from which end you cut the test strip. Or, for that matter whether you cut it before or after spooling it. Personally, what I do is to roll the film onto the spool first and when I get to the end, just clip off the last 1/2" or so before winding the end on. Depending on exactly how you do your unrolling and spooling, the end of the roll may or may not still have the tape (and possibly the paper attached to it). Either way, it doesn't really matter.

Jeff