I had seen on the Star-K website that one may determine if a microwave needs to be kashered by baking a potato in it and then placing one's hand on the roof of the microwave to determine if it's yad soledes bo. If one can hold his hand on the roof longer than fifteen seconds, the microwave does not absorb, and therefore you can use one microwave for chametz and kosher for Pesach.The footnote
Is that measure not subjective? Let's say that my roommate has a higher heat tolerance than I do. The microwave is treif for me but kosher for him, because I can't hold my hand on for the requisite amount of time, but he can? It's for that reason that R' Moshe (OC 4:74) ruled that yad soledes bo is at least 110 degrees Fahrenheit, but certainly at least 160. It needs to be an objective measure, or else you can run into issues like this. How, then, can the Star-K pasken that one may simply place one's hand on the roof of the microwave and see how long he can hold it there?
I have tried reaching out to the Star-K directly for an answer, but being that I have never heard back in any of my attempts I thought I would try my luck here.
To respond to comments regarding whether this is true of kashering microwaves for anything besides Pesach: First of all, if it absorbs, it absorbs, and if it doesn't, it doesn't. We don't say that it absorbs regarding ordinary kashrus but not regarding Pesach. Second, for those who still wish to argue, here is a quote from an email in a separate discussion with the Star-K:
There are differences of opinion as to how to kasher microwaves. I can tell you what Rabbi Heinemann (the posek for the Star-K) holds. Here is a quote from an article on the Star-K website (http://www.star-k.org/articles/articles/338/the-star-k-pesach-kitchen-2/) regarding kashering microwaves for Pesach. The same applies to kashering treif microwaves:
He goes on to quote the article linked above. Q.E.D.