VT and DT females are usually harder to tell apart. HMs are a bit easier but sometimes you can get them mixed up with PK or DT.
I can tell an HM from everything else because that's what I breed and I really don't need to look at the rest much.
For HMs you're pretty much looking for the same caudal qualities you look for in males. A female with a 180 caudal spread and straight caudal edges is a good HM. It's a little hard to count the number of rays so it's better to go with the spread until you get practice.
There was a good article on bettysplendens website about females and dorsal fins but I cannot access the website at the moment.
This is a great picture of a true HM female: http://www.aquabid.com/cgi-bin/aucti...ashm1319237717
Notice the large number of rays on her caudal... she also has split rays in her dorsal which is something that you really want in a breeder female.
I see so many females on AB listed as HM that aren't true HMs, you need to know what you're getting and if you have questions, ask.
When breeding, is very important you look for the best female you can get your hands on... it's very easy to confuse them and end up with undesired tail types in your spawn.
And you SHOULD get a female! They are so much fun!