Replacing headset bearings, or pulling teeth?

[Spinners]

What model of Giant is it? I only ask because I've recently changed the headset cartridge bearings on my 2015 Rapid and found this supplier on ebay;

http://www.ebay.co.uk/itm/GIANT-OD2-OVE ... mLT_PeELHA

[Gattonero]

Wheel bearings are easy, just take a few measurements and go to a website that sells bearings. Headsets tho, there was absolutely nothing written on the bearing I wanted to replace. I ended up emailing Giant, who responded by giving me a link to something I already came across, but wasn't sure of. The bearing I ended up buying was listed for my bike, but for the 2016 version. Could find nothing of the 2015 version which is what my bike is.
Is there any practical way of measuring the bevel angle of a cartridge headset bearing accurately? Other advice on identifying such parts other than searching for make, model and year of bike?

Get verniers and a 45º set square.
Measure the OD of the bearing, it ought to to 41.0, 41.5, 41.8 for normal 1" 1/8 non-tapered headsets.
Tapered headsets can be a bit more tricky, most use 1" 1/2 for the bottom, but there are funny people who use 1" 3/8 and 1" 1/4.
Also, some manufacturers use proprietary bearing sizes, and this you really have to get them from the shops who sell them, and pay £10-15 for a bearing that should cost no more than £5.

Use the set square against the light, to check the angle of the bearing. Most use 45º on the outer.

Unfortunately, integrated headsets could be a simple and effective solution, if wasn't for some manufacturers that are using it wrong, making proprietary sizes to fit their questionable frame designs