Research decloakers. Pretty huge decloaking range, and you can comb a planet with one in about 30 seconds. They'll find your hiding cloakers real fast. Alternately, if it's only a handful of them, you can ignore them and they will probably never cause you trouble. However seeing those red numbers on my planets, even if it's just one unit, drives me crazy. So I always invest in decloakers if I have any cloaked ships at all.
Edit: If it's a lot of cloaked units, be sure to keep your fleet near your decloaker, because as soon as they're revealed, they'll go into attack mode.