Not all HM pins have patterns, some just have a textured back but are stamped with a box that should say what number the HM is out of (ie: 2 of 5), have the official Disney trading logo with the correct year on it and the ever present (C) Disney, CHINA box somewhere as well.
Sometimes it's really hard to tell if a pin is fake or not as some counterfeits can be well made but usually you can tell by general thickness of a pin (most fakes are thinner than their official counterparts), whether the paint is too vibrant or dull or streaky/blotchy, there can be dips in the paint or some places won't be cut out completely where there shouldn't be any metal showing at all. Particularly with the HM pins a good way of telling when a pin might be scrap is by taking a good look at the Hidden Mickey icon somewhere on the pin. A lot of the time on fakes the little Mickey head is malformed; the ears are either way too small or oval or the whole thing looks disproportionate.
If you aren't sure about certain pins, just ask, likewise for not being able to find certain pins on Pinpics. The people here are generally pretty helpful. If you can take pictures of your pins, all the better because that tends to help others see for themselves what issues you might be seeing with your pins.
By the way, welcome to the board and pin collecting and trading. ^_^