I just walk around the room clapping and yelling. I have developed two claps for this purpose. One is the normal low clap and another one is the extra special high clap which is done fingers seperated and is an octave or so above the original clap. I also walk around screaming phrases like "give daddy a bone!" or "kiss my ass!".
After doing that for awhile I can often pick the place to put the drums that is not the worst. Sometimes I can also find great spots for travelling room mics as well.
Just don't put them in the worst place.