The most common way to represent a multiple state selection with a checkbox is to give it a dash. In the example below: all students have "Lorem" selected, some students have "Ipsum" and some don't, nobody has "Dolar".
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In the case of radio buttons your best bet is to just leave them blank. If you really wanted to make sure the mixed nature of the selection is called out, you could put something off to the side.
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Think about what your users are doing, what they task flow is and their ultimate goal in modifying multiple records. Also, what is the field in question and the required nature of that field. Keeping with the student example, every student is a "male" or "female" (at least for this example). It would be assumed that this is a required field too.
A user will have selected multiple students themselves, and entered the edit mode. They know they have multiple students selected from the beginning (and the few fields that have "multiple" in them furthers this). When they reach a set of radio buttons that are blank, it's because they have multiple students and they know it.
Don't underestimate your users. They know how they got here and they know why both radio button might be blank.
If in the case the user is here to make sure everyone is "Male", they can quickly see that is not the case and fix it by selecting the radio button. Now everyone is.