While I generally would discourage writing in stickings on a score, there is a time and place for it. On any instrument, it could be desired to have two drastically different sounds created by different beaters, which would need to be specified. Regardless of what notation is used, it should be specified in your performance notes.
specifying mallets on marimba, vibraphone, etc. using numbers
In 4-mallet playing, mallets are identified by being numbered 1 through 4, with the 1st mallet being the lowest, or in the outer position in the left hand. If the music doesn’t need anything out of the ordinary, specifying mallets is typically unhelpful and does nothing more than crowd the score. Here is an example of a well-meaning but unnecessary sticking:
Once a part starts demanding more specific sounds, specifying mallets can be helpful if not totally necessary. If, for example, you wanted the player to hold an extremely soft yarn mallet in the 1st position, medium yarn mallets in the middle positions, and a hard rubber mallet in the 4th position, then it would be absolutely necessary to clarify which mallet is being used and when.
using different note heads
While numbers could still be used to specify when to use a specific mallet, one might also consider using a different note head altogether. Assuming there are no extended techniques that could cause the notation to be confusing, this could be the clearest option. See the below example, where a passage may require a certain note to be played with a hard mallet in the 4th position.
You could also employ this tactic on other examples. On a drum, for example, you could specify a stick vs. a brush, but that could potentially cause confusion as alternate note heads are typically used to indicate playing position (head vs. rim) rather than beater choice.
using different stems
Certainly not my personal favorite but a still valid method is to write stems in different directions to indicate which hand/beater is to be used. This technique is implored heavily in Elliot Carter’s Eight Pieces for Four Timpani, looking something like the below example:
In this case, the upward stems would indicate one hand, and the downward stems the other. In Carter’s music, this is used in ways such as indicating which notes should be played normally and which should be played with the butt of the stick.