As others have said, do not look back to watch your partner serve or hit a ball in play. However, there are a few times times when you should glance back. When your opponent hits a shot towards your partner that appears that it might land very close to the baseline or sideline, it is a good idea to briefly look back to (help) make the call. Quite often your partner will be more focused on the ball than the line and won't be able to determine exactly where the ball bounces wrt the line. OTOH, if they focus on the line, it might be difficult for them to play the ball. A Catch-22, if you will.
Also, when the ball is in close proximity to our position, it is very difficult to determine its location -- or to even see it all, in many cases. You, as the net player will often have a better perspective on the bounce wrt to the line in question. Many players mistakenly believe that the player closes to the bounce has the best view. The opposite is often true -- of the 4 players on the court, the one closes to the bounce often has the WORST view. This is because they are too close and the ball moves across their field of vision too quickly. For players further from the bounce, the ball moves thru a very small angle -- it does not travel thru their field of vision very quickly at all.
Consider a jet traveling at supersonic speed very high in the sky. Since it is relatively far from us, it does not move across our field of vision all that quickly and we don't have much trouble seeing & tracking it.
Another situation where you MIGHT possibly look back (briefly) is when you think that your partner might somewhat out of position and might not be in a good position to play anything but a defensive or emergency shot. It would be good, for instance, to determine EARLY if your partner is going to put up a lob or play a very weak shot. Only glance back for short time in this situation -- DO NOT WATCH THE BALL COMING OFF THEIR RACKET. Make a determination quickly and then, if needed, back off or adjust your position accordingly.
Otherwise, DO NOT LOOK BACK.