To collect dog tags from nearly all guards, do the following. The only drawback is that it may take some time, depending on where you are in the mission; knocking out all other nearby guards before starting and targeting reporting guards last is recommended. If you cannot get close enough to a guard to demand his dog tags without being spotted, start by tranquilizing him from a distance. Run to where he falls, then pick him up from the shoulders to make sure he is facing up. Position yourself so that you are standing in line with his head (but not literally on top of him). Also, make sure that when he stands up there is enough room in front of him that you can run around and demand his dog tags. When he starts getting up, equip you M9. The moment he reaches a standing position, hold him up. The guard will not have time to turn around and see your character. To wake up guards faster, use either the Coolant Spray technique outlined elsewhere or prop the guard against a wall (feet first, preferably), enter first person view, and punch at the highest part of his body. Both ways will get them up faster, but make sure you are in a good position to hold them up in case they wake up faster than expected. This technique also works well to salvage failed attempts (i.e. you were spotted, panicked, and tranquilized him, which is a good reason never to have your USP or Socom equipped when first holding someone up as you do not risk panicking and killing the guard by accident.)
Go up behind a person and hold them up (point your gun at their back and saying freeze), then go up in front of them then point your gun at their face (the USP or SOCOM recommended). They will say something like "Don't shoot" or "No". They will shake and the dog tags will fall out. After that, shoot them in the head (optional). If they say "Go ahead and shoot me", shoot either their left hand or directly above their head. They will shake and the dog tags will fall out. If they do not shake after that, they do not carry dog tags.
While collecting dog tags and do not know if a guard has one, equip the 'Thermal Goggles'. If he has a dog tag, he will be wearing a necklace with a small metal plate on it. His body is red because it is warm, while the dog tag is blue because it is cold. The best way is to tranquilize the guard and turn him over. When he lays on his belly, and look at the body with the Thermal Goggles.