According to this article, up to two defensive players can have helmets with radios/microphones to communicate with their defensive coaches, starting in the 2008 NFL season. However, only one player can have a radio on the field at any given play. Each of these radio-equipped helmets is marked with a green dot.
As with the offensive transmitters, the defensive devices will go live
immediately after the play clock begins and will remain active until
15 seconds are left on the play clock or the snap of the ball,
whichever comes first.
Two defensive players from each team will be identified and will be
authorized to have receivers in their helmets, one as the primary and
the other as the backup.
The primary player will have one "live" helmet on the field and a
second "live" helmet stored in a secured trunk or container as a
backup in case of a malfunction.
The backup player will wear his regular helmet on the field and will
have a "live" helmet stored in the secured trunk or container in the
event of an injury to the primary player.
The advantage of having these helmets is that the defensive coordinator can make changes to defensive calls, based on what the defensive staff sees from the offensive. This change provided defenses a counter to the quarterbacks' communications advantage. From the offense's perspective, knowing which player is making defensive audibles is probably of limited value, since the radio turns off with 15 seconds left on the play clock. The defensive player with the radio would be quite obvious - the two designated players are chosen before the game, and the active one would be wearing a helmet with the green dot and would be passing along updated defensive plays.
However, there is another possibility, as Peter King notes in a recent Sports Illustrated column. He relates an Eli Manning anecdote describing the importance of knowing who is "the mike":
When quarterbacks go to the line of scrimmage, they most often point
to the foe they're using as the middle linebacker, in order for the
offensive line to know which man they're going to block. The first man
to the right of the "mike'' linebacker, for instance, will be blocked
by the right guard, etc. And so when Manning would see Ray Lewis,
number 52 on the Ravens, across the line and bark out, "52's the
mike,'' Lewis would scurry to the outside of the formation and yell,
"I'm the mike!'' And Ed Reed or another defender would slip into
Lewis' spot and yell, "I'm the mike!'' They were taunting Manning, and
it shook him up.
So according to this column, knowing which defensive player is "the mike" sets up the offensive blocking assignments, which is likely to be of more importance to the offense than knowing the identify of the defensive player with the radio-equipped helmet.
EDIT: Fixed typo in last paragraph.