What signals a braille reader that an abbreviation followed by a period is just that and not a letter followed by a subscript 4? The short answer is "space."
Consider the examples "21 m. superscript 3" versus "21 m subscript 4 superscript 3". In braille, the first will have a blank cell between the 21 and the m., while the second will not. See the attached document for this example presented in a less verbose way.
The slightly longer answer to your question is "sometimes space, sometimes context." For example, "m. superscript 5" and "m subscript 4 superscript 5" without any other content or context have exactly the same braille dot configurations. But for the braille reader, there will always be surrounding content (e.g., in the first example in this message, there is the 21). On the off chance that there is NO surrounding content, just nothingness as far as the fingers can feel, then it is our job as transcribers to do something (i.e., insert a transcriber's note).