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Let us assume there is an workplace environment which has Jewish and non-Jewish employees and there are multiple microwaves, some designated clearly as "for kosher food only" and others that are for any type of food. These microwaves are in a public area which anyone can access.
Under what conditions may a Jew use the 'kosher' designated microwave to warm up food?
This is a rather broad question, and it seems like you're seeking advice rather than a strictly halachic perspective.
In general, if the microwave will definitely be used by only Jews and only kosher food is placed in it and it is designated as being only dairy or only meat, then, there shouldn't be a problem. The main reason that I state "only Jews" is that there could be a problem of "Bishul Akum" even if a non-Jew places kosher food in it, or even a potato.
Practically, though, I wouldn't recommend this in a mixed environment. Most workers, I find, are both careless and "care less". Non-kosher food will likely get into the microwave accidentally or even purposefully. There are loads of Gentile and Jewish anti-Semites who want to cause a problem, and this is an easy way to demonstrate it, and no one will know the difference. After all, when you're not there to observe, they've cooked their food in your oven and eaten it, already. Essentially, it's highly unlikely that anyone there will guarantee that the microwave stays kosher unless a reliable Jew is standing guard watching everything that enters it.
Case in point - someone I know works at Price Waterhouse. He says that on his floor, about 80% of the people are yarmulke / hat and / or "tzitzit-swinging" Jews. Despite that overwhelming majority, they won't put a kosher microwave in their pantry mainly for the reasons I mentioned, as well as that even among religious Jews, no two people have the same kashrut level.
I fully agree with DanF's answer, that unless we know for sure that it's not being used for now kosher food, we should not rely on non-Jews to keep to the rules, either because of carelessness or cluelessness or honest mistakes. I mean, did we ever put a dairy fork in the meaty sink? So if we can make that mistake, kal vachomer...
Therefore, to be safe, I suggest that it be treated as a non-kosher microwave. You'll see that that's not such a hard thing to do.
Below is an article by Rabbi Tzvi Rosen on Star-K's website, titled Microwaving in the workplace. It's about using a non-kosher microwave.
(The full article can be found here)
There are three types of microwaves: Regular microwaves, Microwaves with a browning element, and Convection microwaves.
A browning element microwave or a convection microwave must be treated like a regular oven and food to be heated must be completely double wrapped in leakproof wrapping. For example, Two Ziploc bags or Saran Wrap.
Since the chamber of a regular microwave does not get hot, food can be warmed uncovered if the following criteria are met:
The microwave – the ceiling, floor, and door and walls are completely clean of food particles, spills or residue.
No non kosher food is being microwaved in the microwave at the same time.
The floor or turntable is covered or the food is placed on a thick plate.
Q. What qualifies as adequate floor covering or thick enough plate?
A. A sheet of styrofoam or a styrofoam plate is sufficient.
Q. Can food be warmed double wrapped in a regular microwave if a person does not want to follow the cleaning criteria?
A. Yes
