Answer by Rory Young:
I can only answer in terms of African elephants.
There are three main things I look for:
Firstly seepage from the temporal gland. Although generally a sign of stress in elephants, in bulls in musth the gland will also be swollen and the seepage looks thinner.
Secondly I look for wet back legs and wet penis and sheath. The bull will seep urine and semen continuously onto the back legs. You can also often smell it.
Lastly I look at the attitude of the animal. Often they can be extremely aggressive but at the very least they will have a wide eye, showing a lot of the sclera, as though afraid, walk with their head held very high and they will take long strides. They also spend more time walking like this and less time feeding.
Once you have confirmed these points are all present get out of there!