Question: How can one tell when his tefillin shel rosh needs adjustment?

Answer: Firstly, an adult who has not adjusted his tefillin shel rosh’s knot in several years almost certainly needs an adjustment. Tefillin straps stretch slowly as we apply pressure to them (some more than others) when fastening the tefillin on our head. If one’s hairline has not receded, he can easily check.
Now, a little review of the anatomy of a normal human head. The skull is highest towards the back of the head; it then gradually slopes down. Near the front of the head, the slope increases, and then turns into a "cliff" (i.e., the forehead). The hairline ends at the end of the gradual or the midst of the increased slope. No hair (except eyebrows) is rooted in the forehead.
Based on the above, the following are signs of misplaced tefillin. If the end of the tefillin looks like it is "hanging off a cliff," it is certainly much too far forward, as a line drawn down from the end of the tefillin would hit the forehead or even the nose. Because of the increased slope, there may be a little space between the bottom of the tefillin and the head. However, if there is too much room (i.e., a finger fits in comfortably), it is very likely not in the right place.
Another sign is the tefillin’s angle. The angle is determined primarily by where the tefillin are fastened to the head by the straps – at the back of the tefillin. Generally, tefillin in the right place will be upright with a slight downward slant. If the tefillin has a serious downward-facing angle, it is generally (unless one has a rounder head than most) too far forward, so that its rear is where its forward part should be (on the steeper slope). Thus the tefillin’s front will be too far forward, unless the tefillin are very small.

Bemare Habazak - Rabbis Questions (510)

Rabbi Daniel Mann

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A final sign is the kippa. With average size kippot and tefillin, there should be little or no room between the two. One with a particularly large kippa or who wears it on the top of the head (as opposed to part top/part back) will have to move the kippa back.
When I look around many of the shuls I regularly daven in or visit, I see many too many people with apparent (or definite) problems in this regard. Among the older generation, I would estimate that the problems are in well above 50% of the people. As I HATE correcting people (and most hate being corrected), I am torn as to when the rectifiable problem is clear enough to halachically/morally require me to do the uncomfortable. The following limud zechut decreases the problem. Most people put the tefillin at a certain position and push it forward in the process of fastening. Thus, some of those who keep the tefillin too far forward had it in the right place for a few moments after the beracha (so that it is not l’vatala) before the fastening was complete, and thereby may have fulfilled the mitzva for that short time.
More people should learn how to shorten the circumference of the head strap, which is necessary for the tefillin to stay in the right place. You are invited to visit me or ask a sofer. It may be easier to Google search: "youtube tefillin head adjust." Then, you can help yourself and your friends.