I use my thumb.
Really, I use my thumb to help visualize the entry. It's not my idea either. I think it's Ralph Butchers, but Ron will know.
Take your thumb a place it gently on top of the 90 mark on your DG or HSI. Which thumb? Left thumb for left turn patterns or your right thumb for right turns.
Now that you got your thumb out, mentally draw a straight line from the top of your thumb, through the center of the DG or HSI. Mentally draw a line from the top of the instrument to the center. You now have divided the 360 degrees into three segments. The first is the 180 degree arc and this represents a direct entry. The wedge shaped one that looks like a tear is the area where you'd do a teardrop entry. The last area is the parallel entry.
So, find the inbound leg of the holding pattern and see what segment it falls under. That will tell you the entry.
Sorry, it's so much better with pictures or better yet a whiteboard and trying various holds.