Hi everyone - this is probably a real novice question but I was hoping someone could shine a bit of light on a problem I'm having regarding headset bearings.

I've followed a youtube video showing how to replace bearings in the headset, and it's straightforward enough until I get to the part where you remove the bearings themselves to ID what type of bearings you have, basically the bearings in my headset are unsealed, so there's no part number or details on them anywhere. As this is the case how can I tell what size / angle I need to replace them with? Is there a standard type that fits most bikes or do I need an identical replacement?

FYI my bike is a 2009 Marin Lucas Valley with a 1 1/8th" threadless headset....

Any help greatly appreciated!