Okay so I am going to go off a whim here and talk about lizards today. Only because this just came up recently. So i have two leopard geckos and I love them. They are very dear to me. They both live in about a 40 gallon tank together and have seem perfectly content with each other, and that is because I have two females. Now if I had two males that would be a different story. Now you may be wondering how can you tell a male gecko apart from a female. Well it is actually fairly simple. Leopard geckos are as many may know dessert geckos. They live out in the Afghanistan areas and in parts of Asia. They do have tails that can come off when they feel threatened or stressed. So I warn you before you try seeing if your gecko is male or female make sure it is comfortable with being handled. Male leopard geckos will simply have preanal pores (there is a link at the bottom of the article to a useful website for further images). These pores are much, much harder to see on females if seen at all. The males will also have two bigger lumps in the postanal section (so where their tail and anal groove meet). These preanal pores are there for territorial purposes. A male leaves his scent to attract females and to alert other males that he is around and that he gets all the ladies. The bulges at the base of tails on males contain sexual organs most lizards have called, hemipenes (these bulges would sometimes be called hemipenal bulges). Leopard geckos are fascinating little land geckos. One interesting fact about them is that their gender is sometimes impacted when they are being incubated. Lower temps of incubation will lead to more females. Where as higher temps will lead to males. For details on this I will also provide a link.

Here is a picture of one of my females. notice if you look carefully you can see her "V" shaped preanal scales but no apparent pores, and she does not have as big as lumps (hemipenal bulges) below her anal like most males would.

Here are the links:

http://www.reptilecare.com/leopardgender.htm

http://www.geckodaddy.com/blogs/news/1727752-sexing-leopard-geckos

Many lizards are similar when it comes to identifying male or female, but here are a few explanations for other lizards just in case you know a friend or someone who may be interested.
(So maybe you have a bearded dragon or some other type of lizard you would like sexed well here is some advice.)

Bearded Dragons: many say that males tend to only flash a black beard. Well that way may work for some but the best way to see if it is actually a male or a female is to gently lift its tail and on its tail you will see either two bumps, one bump, or no bulge at all. If you see two bumps then your looking at two hemipenal bulges so you have a male. If you see one small bulge at the very base of the tale or none at all then you have a female
Monitors: once again a big give away is the hemipenal pores on males. for some species like the Savannah monitor, red dots are around the preanal area on females. Males also tend to distribute more territorial actions from time to time.
Chameleons:The males will within a few months of growth have a hemipenal bulge at the base of the tail. Females will be smooth. For most species males tend to be larger and most males contain tarsal spurs on their hinds legs ( look like barbs)
Iguanas: these guys have femoral pores on their hind legs and the males are very easy to tell apart from the females. They males contain very strong muscular jaws and they also contain several fat deposits in the head. Both of these factors add to the males bulkiness and size. opposed to the females who have a more narrow and lean head and body. Of course the females femoral pores are not near as significant as the males.
Chinese Water Dragons: So similar to the iguanas the males have more protruding femoral pores (similar to preanal pores only located in the hind legs.) the males on a chinese waterdragon, when fully grown, will have nice beautiful long spikes down their spine and have an easily seen bony, cartilage like vertebrae that should be easily seen. The females do not have as apparent vertebrae. Males a broader and bulkier look.
Other geckos: most geckos have sexual characteristics related to each other, such as the preanal pores and hemipenal bulges as i mentioned before. Many kinds of geckos have barbs that assist in breeding as well. Of course the males will have bigger barbs then the females. Males bodies and head are stouter and bulkier in shape and size
Hope the information helps all you lizard lovers out there! I have links at the bottom of the page where I obtained some of my information. So look for those hemipenal bulges they're the biggest give away, and feel free too comment below if you have any questions. I would be interested to know what kind of lizard you have and if I helped you identify it at all! Thanks feel free to follow and subscribe!

http://animals.mom.me/tell-difference-between-boy-girl-geckos-6735.html
http://www.triciaswaterdragon.com/sex.htm
http://www.greenigsociety.org/malefemale.htm
http://www.wikihow.com/Tell-if-a-Chameleon-Is-Male-or-Female
http://animals.mom.me/savannah-monitor-boy-girl-10592.html
https://www.beardeddragon.org/articles/sexing/