If your bike is this: 2008 2.3 WSD - Bike Archive - Trek Bicycle

Then the headset is listed as an Aheadset semi-cartridge. Aheadset: AHEADSET :: Bicycle Headset Manufacturers | Bicycle Headset Assembly Videos

You can eyeball those and the Park tools site for info. These are not particularly challenging to R&R. You can clean and relube
all the contact points between the headset and the frame and see how the bearings feel when removed. Cartridge bearings in
low load scenarios like headsets should last forever, however the grease can dry out over time and make the bearing noisy or
stiff. You can pop out the plastic bearing seal and clean out and repack the bearing with grease and replace the seal fairly easily.
Tracking down sqeaks can be difficult. You should also consider seatpost/seat tube interface and sometimes the saddle clamps
and saddle rails can be noisy.