If you have an RMI or HSI, you can use the finger method using the right hand for standard right holds, or left hand for non-standard. Thumb, pointer and middle fingers spread out and laid on the face of the HSI with pointer finger pointing inbound on the holding radial......sector between the thumb and pointer is a parallel, sector between middle and pointer is TD, and then anything outside of it (like the back of your hand) is a direct. It is a rough estimate, so you can't really gnat's a** it, but it will get you in the ballpark quickly if you have no idea.
BTW I'm not a CFI, just hadn't heard anyone mention this technique and it has helped me in a pinch when time is of the essence.
EDIT: nevermind, VTFlyer already had this one covered. So +1 to him!