Trying to identify headsets I need to replace the headset on my old trek 1.5 alpha 2011 model, and also my b'twin ultra 720AF will need the bearings replaced soonish.

Is there a reliable way to find out what headsets I need? I've had two close, but unsuccessful attempts at getting the right ones before. In the end I've had to give up and use my lbs, which is annoying as I have all the tools required to change these.

Any advice is most welcome.
I sent trek an email and they responded . They say the correct headset has article number 423140 and costs 29.99€