Perhaps you could ask them to report at a geographic landmark, if available. That would make it easier to confirm their direction of approach, if you didn't have radar. For example, I usually make my initial call 10 nm out to Class D airport. Sometimes, the tower will ask me to report 3 nm out. Sometimes, the tower will ask me to "report hospital" (about 3 nm out). Obviously, this depends on their approach, and the geographic landmark would have to be clearly marked on the maps.
If you really aren't sure, make the pilot give more information. Request location, altitude, speed, and heading. Most pilots only report location and altitude on their initial call, so requesting more information from them might make them think "oops - I gave the wrong info on my first call" and they'll fix it when they reply. If the pilot isn't sure or you get a lot of "uh... er... well... stand by," make them come back when they know what they are doing (keeps the rest of us safe).
Another thing that might be confusing is how the call is made. If you have a preference, it wouldn't hurt to contact the local R-22 schools and let them know what the guys in the tower like to hear. Better yet, sponsor a class once a quarter and invite the students at these schools and all local helicopter pilots. I usually go out of my way to attend an
FAA Wings Safety Seminar if it is specific to rotorcraft (not very common for some reason).
This is an interesting thread to me, and I wonder what the rest of the
Rotorheads group does. I usually report my location as reference to my destination, and I rarely give my heading unless requested. It's usually something like, "XXX Tower, Helicopter XXX 10 miles east at 3000, landing with Lima" (if flying an R-22, I'm usually 85-90 KIAS heading 275 in this scenario, but I usually don't report that).
Steve76's acronym is better!