If you don’t have an Experflyer subscription, just do a search for your flight and check any one of the ‘upgrade fare type’ buttons (i.e Gold upgrade, Gold Guest upgrade etc.). If the box with the F is solid blue with a white F, there is U space for the number of seats you searched for. So if you need 2 U seats, make sure you search for 2 pax. If the F box is a white background with a blue F, there is no U space.

The upgrade type doesn’t matter if you are just looking to see if there is U space. A U is a U, regardless of which method you use to upgrade. You just need to ensure you have bought the correct fare class for the method you plan to use to upgrade. But if you just want to know if U space is available, it makes no difference which box you check.