I promise you that your debuff frame will show you all the slows on you. As for an addon that shows just slows? I guess you could make a weak aura for every slow in the game...
If you use TellMeWhen it actually has a preset to TRACK slows though it'll only show one at a time but it's guaranteed to have the icon of at least one (I think it picks the one lasting longer?). I use it on my warrior to track hamstring (or any slow) for Staggering Shout so it's just a matter of setting up to track on yourself.