How can I identify alkene?

Hint: As we know that hydrocarbons can be saturated and unsaturated compounds of carbon and hydrogen and are named as alkane, alkene and alkyne. These hydrocarbons may contain a single bond between two carbon atoms, a double bond as well as a triple bond.

Complete step-by-step answer:
We already know that according to IUPAC naming culture, the alkanes are defined as the saturated hydrocarbon compounds which possess only a single bond between carbon-carbon atoms. The alkene on the other hand are those compounds which contain a double bond and ends with the suffix ‘ene’ and alkynes are those compounds that contain a triple bond in them and ends with the suffix ‘yne’. Both alkene and alkynes are thus unsaturated.

 Alkenes are represented with the general formula ${C_n}{H_{2n}}$. Similarly, alkynes are also unsaturated hydrocarbons possessing the general formula as ${C_n}{H_{2n - 2}}$.

Now, we know that the alkenes and alkynes are generally named by identifying their longest chain of carbon which is termed as the parent chain and in case of alkene, the parent chain usually contains a double bond whereas in case of alkynes, the parent chain contains a triple bond.

Actually, the parent chain is numbered in such a way that we would reach the double bond or the triple bonded carbon atoms the earliest. Let us take an example of an alkene:
$C{H_3}CH = CHC{H_2}C{H_3}$

So this alkene will be named as $Pent - 2 - ene$ starting the number of carbon from the left hand side so that we will reach the double bond earliest and the position of double bond is indicated by the number $2$.

Then, a suffix ‘ene’ is added to the parent chain in case of alkenes while naming and suffix ‘yne’ is added to the parent chain of alkynes. For example: ${H_2}C = C{H_2}$ is a typical alkene and is known as Ethene and $HC \equiv CH$ is a typical alkyne named as Ethylene.

Note: Always remember that the longest chain is always numbered from that end which would provide us with the lowest number to the carbon atom where the double bond or the triple bond is present in case of unsaturated hydrocarbons.