Determining the year of your barbecue depends on the make, model and the condition of the grill. Normally all the specifications are on the cover (front or back) of the users manual.
If the manual is not available the manufacturer should have a indoor/outdoor sticker or a steel panel riveted to the drip pan or back side of the barbeque grill.
Take the drip tray out look at the top and bottom. If it is not there it should be on one of the back panels normally not viewed.
On a built in grill the grill head may have to be slid forward from its place in the outdoor kitchen to view the tag and label.
Legally gas grills must have a "CSA" label with a star in a circle symbol but some imports did not always follow the law because claims of certain materials (304 stainless) were made that were untrue in the warranty. Most of these were sanctioned with fines and attempted to retroactively apply labels by tracking the barbecues.
The CSA label should have the model number - usually several model numbers depicting various options of that model and year - and the serial number. If the year is not clearly depicted it is usually written within the serial number.
A lot of import companies have no one to contact because the branding does not actually refer to a company. For instance 8 years ago Charmglow was owned by HomeDepot but imported through Nexgrill from an unknown manufacturer overseas.
In this case knowing the year will probably help nothing because there is no way to use a warranty anyway. Replacement parts can be determined by the model, by specifications of the actual parts or contacting someone who knows the products like Grill-Repair.com.