Each to his own, but those are bad methods which will leave you with terrible topology. I personally start low poly and add detail, however if you start high then retopo a lowpoly mesh (with proper topology) and then bake the normal map.
I do the same as Excalaberr. first create a low poly then build up detail for a high poly version. the only difference is when the high polys done I use the shrink-wrap modifier on the low poly version so they match up perfectly.
You don’t need a tool. The polygon count will be displayd in the top menu bar on the right side. When you select the object and go in to the edit mode then only the polygon cout from this object will be displayd.