Steve could've been wearing a rug, but he always seemed to pride himself by his hair every time I saw him. He'd play it up, but not like someone that lost their hair X amount of time ago and was holding onto his past.
Mandel I think really did shave his head. Never had any signs of balding that I remember, not that I tracked his career or anything.
Usually, you can see a delineation between the thinning hair and the sides after a couple of days goes by without shaving it. Some people can get away with more time, some less.
After 3-4 days my hairline shows that I'm thin on top.
Stars like Howie and what not, can usually get their heads shaved every day or two, and by a professional on the set. I know Howie endorses the HeadBlade, but I'm willing to guess that he has it done by a straight edge every day or two because he's constantly filming something like a commercial, tv show or whatever.