It depends on the style. If it's realism, you could just draw the skin looking shiny. And it would be like drawing water drops running down a surface. If it's anime, you just draw a big old drop shape on their head.
well, he's not nervous or anything, he's in a fight. the anime teardrop wont really work out in this scene. it's too hilarious. ive checked bing and stuff for pics, but the problem is that i dont know how they did what they did. any advice?