Summarize the following:
A male plant, compared to a female plant of the same strain, generally has a thicker stalk. That is because it gets taller than female plants and needs to be able to support the weight. They also have fewer leaves than female plants. If male plants are allowed to pollinate, they will severely lower the potential crop of your female plants. That's because, once fertilized, female plants spend energy producing seeds instead of THC, leading to a smaller harvest. If you are growing indoors you will likely be visiting your plants regularly, so the following checks should be easy to do.  You need to check every plant to determine if it is male or female, as one rogue male can wreck your harvest. In general, male plants show their sex 7-10 days (indoor) or 3 weeks (outdoor) before female plants. The little balls that grow on the joints of the stalk (where the other branches meet the main stalk) are the main indicators of male plants. These flowers release pollen and need to be removed for a better crop.  If you're trying to create new plants or reproduce, you need to leave these balls undisturbed. Female plants will have these bulbs too, but will also have long, translucent hairs on them. If you only see 1-2 on a plant, wait and see if more develop before cutting them. Marijuana plants can grow both sex organs. If you see any of the tell-tale male buds, you should trim them like you would a normal male plant. They will still release pollen that can ruin your crop. "Hermies" are generally undesirable plants, and they can ruin a small crop with their pollen if you're not careful. Once you've determined a plan is male, you need to get rid of it or it will ruin your crop. Do not try and remove the buds by hand, as missing even a few will significantly decrease your crop. While most growers simply throw the plants out, a few keep them around for breeding purposes. If you do, put them in a separate room from the females, and make sure you don't track pollen in from the male room to the female room on your clothes or hands.

Summary:
Look thicker, sturdier stalks with fewer leaves on male plants. Check your plants regularly from July to September to sex them. Check the joints on the stalk for male flowers. Know that hermaphroditic (both sexes) plants exist, and should be treated as males. Throw out or remove male plants unless you specifically want seeds.