if your talking about a tear running down a face (or cheek) you might try working with cc glue gun.
you'll want to duplicate the face layer and add cc glue gun to the top duplicate. you'll need to key frame the brush position property to move down the face (this may be the hardest part to make look real) and adjust the stroke width.
you can add some randomness to the 'tear' by setting the style to wobbly and adjusting those properties. also play with the light settings (i find point is often better than distant source) and the shading settings.
you can also adjust the opacity of that layer over the original layer if needed. you could also add roughen edges to the tear if wanted more irregularity to the tear path.