Concerning the twelve marks of a sadhu: hand applied, by every yogi, it would be a cause of concern if any marks were to appear to be 'perfectly applied' by using a mirror. Those marks are applied during the sanctified bathing, and must never be applied with using a mirror, nor even by looking at oneself in the reflection of their water pot.
Also for the yogis who apply 'ash', who usually claim to be serving a delineation of a Rudra attitude, that those marks which appear as three lines of white ash can never be 'straight across the forehead horizontally'. If the ash mark appears as three equal width lines, applied with open three fingers, then the sadhu is probably an impostor, and should be ignored. Sometimes the actual mark may appear to be some sort of messy smear where a simile of three lines appear to be. The vertical 'U-shaped' marks of devotional service vary in a large variety, but they must never have any lower 'stem' to the 'U' such as to make it a 'Y'. It may have a series of marks inside the U, and a dot below the U, as also the ash marks may also have marks centered within it, and a dot below it. It is also permissible for the sadhu to apply such marks with only 'water', and thus nothing would be seen remaining. You can ask those who appear without marks, as to what type, and in what manner they apply their devotional service designations, if you are not sure about this. This latter mark seems to me, to be cosmetic, but it is apparently, one of the primary credentials to be validated in a Sadhu, that is, a 'Good Fellow'. Next - All About Yogis.
