If you're just in trail of a plane you can see when the sink or drains are utilized. The right sun angle and positioning helps to see it. Less common, I've only seen it once or twice in 25,000+ hours, is seeing the condensation cloud from lift being generated at altitude. I saw it once from the horizontal stabilizers of a SW 737 1,000' above us heading the same degree. They were up sun of us and barely skimming a very thin cirrus layer. In this case the 'lift' cloud was on the bottom side of the stabilizer. Memorable to see when you've only seen it once.