Kind of a fuzzy question, maybe you could clarify it. If you're trying to detect whether a unit has a radio signal available, then you might use a variable:
When adding the message:
1 setradiomsg "Do Something"
signal = 1
When removing the message:
1 setradiomsg "null"
signal = 0
That way, whenever you want to detect if a unit has the message available, you just check to see what the value of the variable signal is.