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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.

DonielF
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    Conjecture: Whatever temperature the Star-K holds yad soledes bo is, no one can hold his hand against plastic that starts off at about that temperature for fifteen seconds. – msh210 Jun 19 '17 at 07:00
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    Did you try to phone directly? – sabbahillel Jun 19 '17 at 10:25
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    @msh210 Not all microwaves are plastic. I don't know what mine is made of, but it passes the potato test. – DonielF Jun 19 '17 at 11:34
  • The Star-K does not say that you can use a microwave oven for both milk and meat at this link. It is only discussing kashering for Pesach and says this is a way to test if the power of the microwave causes the surfaces inside to reach the point of 'Yad soledet bo', hot enough that one would have to remove their hand because of too much heat. This is only in the context of whether the surface of the walls of the microwave absorb the chometz preventing its use for Pesach. It also emphasizes that this doesn't apply to the turntable which actually comes in contact with hot chametz. – Yaacov Deane Jun 19 '17 at 20:30
  • Presumably there's already a siyag built into the test that accommodates some subjectivity. – Isaac Kotlicky Jun 19 '17 at 21:21
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    @Yaacov Fair, but why would it be any different? Either it absorbs or it doesn't. – DonielF Jun 19 '17 at 23:01
  • @IsaacKotlicky Obviously, I just can't figure out what it is. – DonielF Jun 19 '17 at 23:02
  • I am pretty certain that @msh210's conjecture is correct. If it's yad soledes bo then you certainly won't be able to hold your hand there regardless of your tolerance for heat. If for some reason you have no feeling in your hand and you are completely unaffected by heat then I suggest this test doesn't apply to you and that fact wasn't included in the article because it's obvious. – Daniel Jun 20 '17 at 03:31
  • The article from Rav Heinneman is again only discussing for Pesach. His comment about being able to change from milk to meat or vice-versa is only in regard to keilim (knives, forks, spoons, etc) that have undergone full kashering through hagoleh. He does not use this language in regard to ovens, stove tops, etc. – Yaacov Deane Jun 20 '17 at 10:48
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    @YaacovDeane Thats because ovens and stove tops require libun, not hagalah. Microwaves don't get hot enough to require libun. – DonielF Jun 21 '17 at 00:21

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