How can you recognize alkenes?
Put simply, any hydrocarbon with at least one double-bonded carbon is considered an alkene.
Let's look at a couple different hydrocarbons and see if we can classify them.
We'll look at Hydrocarbon 1 first. It's got four single-bonded carbons, so this guy is known as butane. Butane would be considered an alkane, not an alkene, since it has no double-bonded carbons.
Hydrocarbon 2 has two carbons, and one double-bond. We call this guy ethene, or ethylene. You can probably guess that ethene is an alkene, due to the "ene" and you'd be correct.
Hydrocarbon 3 has three carbons, and there is one double bond. We'd call this guy 1-propene, or propylene, and it is in fact an alkene.
So that's essentially how you do it: look for double-bonded carbons. If there is at least one, the molecule is considered an alkene.