One important point is to let the projector work for a while and heat up to its working temperature before doing the final adjustment focus. There can be some heat expansion in the optical block and the lens that can change from cold to warm. If the 3 chip projector have a convergence adjustment, that can also be of importance for how we experience the sharpness/focus.
Projectors often have a hard time to be as optical sharp in the corners than in the middle of the picture, but the physical setup and angles the projector to the screen etc, is important to get right to have a optimal picture.
There is no particular need to use special test materials, doing the "mechanical" focus, but for setting up how the picture is processed in the projector etc, that can be help full. How sharp the pixels is made to look at the screen is the most important part of the optical focus, but to be able to see one or two of the tree RGB colors at a time may be useful too.
On digital projectors without the possibility to remote control the lens focus , some of best way i know to focus on to the pixel grid itself is to.
1. have someone to adjust the focus while you look closely at the pixels on the screen. (almost like the old areal TV antenna style....)
2. Manually adjust the focus a little at the time and walk up to the screen and check. To check if the focus is optimal, use a white A4 paper and move it from the screen and closer to the projector to see if the sharpest focus point is at the screen surface.
3. use binoculars like jfinnie pointed out.
Used that last time I installed my A-lens. (Binoculars and A4 paper was also often used with CRT projectors that have many layers of focus pr color and is many times harder and complex to focus than one lens focus projectors of today's standard)
4. Use a DSLR camera on a tripod that uses live view and set it to real time show a small area of the projector screen and gives a very sharp and clear view of the focus in that area..
Some 3 chip projectors can have a little difference of the 3 RGB panels that get the "perfect" focus/convergence at the same time. The most important colors to see the focus/sharpness from is the green/red ones if there is differences and we want to prioritize.