GC chromatograms often already have a library of compounds to determine what the substance is. In case the compound cannot be found in the library, one common alternative scientists use when performing is spiking.
Spiking involves gradually increasing the concentration of one specific compound, and looking for a rise in peak height. Here, you can identify which peak corresponds to the compound you spiked. You can do this for each alkene, to determine its peak and retention time.
