Crossplay - Dressing up as a character of the opposite sex. Boy becomes girl. Girl becomes boy.
Genderbending - Reimagining how a character might look & accessorize if they were of the opposite sex. Tony Stark becomes female "Antonia Stark."
Gijinka - Dressing up as something non-human.
4. Is the person wearing heavily contoured makeup?
If the answer is yes, then they are likely crossplaying. You may have to be in close proximity to the person to pick up on makeup details because from far away their features may look like those of the opposite gender.
Warning: This can also be true for genderbending and gijinka.
Mindfuck factor: very high
3. Does the costume have identifying pieces reminiscent of a certain character, but in the wrong gender?
If yes, you may be looking at a genderbent cosplay. Genderbending is taking a character of the your opposite gender and redesigning its clothes & accessories to fit your gender.
Examples: VI of League of Legends (female character) redesigned for a male to wear. Instead of the top she wears, a male might choose to go shirtless. Instead of the female hairstyle/wig, a male might color their short hair. Other identifying characteristics would still be there - like VI's mega-fists, the tattoo on the cheek, etc.
2. Is it sexy/skimpy?
The likeliness of this being gender bending is moderate-to high. Because of the freedom to re-imagine a character's outfit to fit our own gender, people of both genders can take the freedoms to make these costumes super sexy! For example a genderbending male can go shirtless and wear short-shorts, just as a genderbending female can wear a bikini top and bottom as long as it has additional identifying characteristics!
1. Is it human?
You see someone in costume as a NON-HUMAN characer, an object, or embodying an idea or concept: You are looking at a Gijinka! Probably one of the easiest sub-categories to identify since it answers "no" to one basic question: "Is it human?"
Popular examples: Pokemon, people dressed as the Tardis, Daleks, or Glen Corral who did the Johnny Depp ultimate cosplay. This last one is important because even though Johnny Depp is a human, yes, what Corral was cosplaying was the concept of meshing a ton of Depp's characters.
It can be pretty damn hard to distinguish between crossplay and genderbending sometimes, especially if the person is paying special attention to detail when it comes to altering their appearance and mannerisms. If you're still confused, ask them! (Nicely, of course.)