The first rule of thumb is that someone who has not adjusted the knot of his tefillin shel rosh in several years almost certainly needs to adjust it now. Tefillin straps stretch slowly, as pressure is applied to them when the tefillin is fastened to the head (some apply more pressure than others). If one’s hairline has not receded, he can easily check to see if that is indeed the case, and he should make sure that the tefillin rests on the head in such a way that the front of the box does not descend below the hairline..
What should one do if his original hairline is no longer recognizable? Let’s take a quick layman’s look at the anatomy of a normal human head. The skull is highest towards the middle (or slightly towards the back) of the head; going forward, the skull gradually slopes down. Near the front of the head, the slope increases, and then turns into a “cliff-like” change of direction (i.e., the forehead). The hairline usually ends somewhere in the increased downward slope. Normally, no hair (except eyebrows) is rooted in the forehead (see illustration #1).
Based on the above, the following are signs of incorrectly placed tefillin. If the end of the tefillin shel rosh looks like it is “hanging off a cliff,” it is certainly much too far forward, as a line drawn downwards from the end of the tefillin would hit the forehead, and possibly even the nose (see illustration #2). Because of the head’s increased slope, there may be a little space between the bottom of the tefillin and the head, even when the tefillin is stationed in the correct location. However, if there is too much room (e.g., if a finger fits in comfortably), the tefillin is very likely not in the right place (see illustration #3).
Another sign of misplaced tefillin is the tefillin’s angle. The angle is determined primarily by where the tefillin is fastened to the head by the straps that extend from the back of the tefillin. Generally, tefillin positioned in the correct place will be basically upright, with a slight downward slant. If the tefillin has a serious downward-facing slant, it is generally too far forward (unless one has a rounder head than most), such that its front is too far along the precipitous slope.
A final sign of misplaced tefillin relates to one’s kippa. With average-sized kippot and tefillin, there should be little or no room between the front of the former and the back of the latter. One with a particularly large kippa or one who wears it on the top section of his head (as opposed to being partly on the top and partly on the back of the head) will have to move his kippa back to put the tefillin on properly.
When I look around in many of the shuls where I regularly daven or visit, I see too many people with apparent (or definite) problems in this regard. Among the older generation, I would estimate that well above 50% of the people have their tefillin too far forward. As I hate correcting people (not to mention the fact that most people hate being corrected), I am torn regarding the dilemma of in which situations the rectifiable problem is clear enough to halachically/morally require me to do the uncomfortable.[1] The following limud zechut[2] possibly lessens the problem. When most people don their tefillin, they first put it on their heads at a certain, temporary position and only then push it forward in the process of fastening it. Thus, many of those who wear the tefillin too far forward had it in the right place for a moment after the beracha was recited and before they fastened it (and therefore the beracha was not l’vatala), and they thereby may have fulfilled the mitzva for that short time.[3]
More people should learn how to shorten the circumference of the head strap, which is often necessary for the tefillin to stay in the right place. You are invited to visit me or a sofer for help. (It may be easier to search online for “youtube tefillin head adjust.”) Then, you can help yourself and your friends do it yourselves.