Some brands offer an online warranty check so first try links on our Warranty check page.
If an online check does not help you'll want to start by finding out when your unit was produced. There's usually a visible production label on the machine - the placement of it varies by brand and model. Once you have that, look up the manufacturer's warranty and see if yours falls within eligible dates. Note that some HVAC companies offer their own no-lemon and installation guarantees that go beyond what the factory offers.