One thing I was told by my IR instructor which simplified things (although you should still really work it all out, but at least you'll have a good idea in your head whilst you do so) is this:
Imagine flying to the hold.
If your flight will take you to the beacon and out of the hold area, it'll probably be a parallel entry.
If your flight will take you into the hold area then it'll probably be a teardrop.
Direct entries are fairly easy to spot.
Try drawing a couple of examples and you'll see what I mean.
It helps when you've only a minute or two to work it out.