The month of Elul is already here, and aside from the minhag to say Selichot and the obligation to do Teshuva, there is also a minhag to check our Mezuzot and Tefillin.
Although it is important to have them checked by an expert Sofer, there are several things one can check himself.
Checking The Batim
For example, people are mistaken to think that the most important aspect of the Tefillin is the quality of the scrolls in the Batim (boxes), thus paying no attention to the Batim or the straps. But the truth is that the Batim and straps are equally as important, and if they aren’t kosher the entire Tefilin is considered pasul and the Berachot recited over them are void (levatala). Therefore, one should check his straps to make sure they are entirely black. If he finds white spots on the straps, they might be pasul (invalid), and must be painted or replaced. Another concern is a rip/tear in the straps, or if the straps were stretched to the point that they no longer have the minimum measurement (on the width) which can be problematic.
One should also make sure that his straps aren’t peeling which could be a cause for concern.
When checking the Batim, we should see that they’re completely black and perfectly square. The Batim must be perfectly square at both the top and bottom, and the stiches around it must be perfectly square as well. In the event that they aren’t perfectly square, the Tefillin may be rendered pasul. This is more likely to be a problem at the Tefillin shel rosh (the Batim worn on the head) as they’re made of four separate compartments and may sometimes separate, resulting in the loss of the square shape.
Proper Placement of Tefilin
Another very important concern one should pay close attention to is the place we don the Tefillin; one must make sure his Tefillin are placed above his hairline (the line where his hair starts coming out). If they’re on his forehead even just a bit, it’s considered as if he isn’t wearing the Tefillin at all. The Tefillin wrapped around the hand must be placed about two fingers above his elbow (when the hand is at a 90-degree angle). If placed too low or too high, it may be problematic.
The halacha says that sometimes a letter “י” looks like a regular “י” but since the left side goes down very low it resembles a small letter “ח” and is therefore passul and can’t be corrected. Although if one wrote such a “ח” a regular size it would have been considered a kosher “ח”, nonetheless, since it is small, this would still be good for a “י”. Meaning to say, if a “י” appears exactly as another letter, it’s passul. If it’s slightly different, then the small size plays a role and the letter is considered a “י”.
 משנה ברורה במשנת סופרים אות יו״ד
 בית שלמה ח״ב יו״ד סי’ קנו בתשובה לרב שלמה קלוגר בשו״ת סת״ם, וראה לאגר״מ או״ח ח״א סי’ יא
 יריעות שלמה כלל ח אות ב