TMX grain is very fine. Even with a good magnifier, a large negative, such as 4” x 5”, results in very small grain as presented to the eye by the magnifier.
The grain is easiest to see with the aperture on the enlarger lens closed at least one stop. This seems to give me the greatest contrast in the grain itself when viewed with my Bestwell 25X magnifier. The best practice is to close down to the aperture at which you print and then use the magnifier.
One way to overcome the problem is to choose a different target. Instead of focusing on the grain, focus on some small detail with strong contrast, such as a dark detail against a much lighter background. For example, you could focus on the junction between a shadow (light on the negative) and a white area (nearly black on the negative). This is as accurate as using the grain as a focusing target and can be much easier to see.
Those of us that used the now-discontinued Kodak Technical Pan film had an even harder time seeing its grain, as it was finer than that of TMX, especially with 4” x 5” or larger film.