I'm re-furbing an old beach cruiser and need to buy e new headset. How do I tell what size mine is? I cant see any obvious markings on it and don't know where to measure it.

It's probably a 1" threaded headset.

Need to go to your lbs and let them measure it. It may be 1", but the width varies a lot. They can look through their books for a match. Seen too many times people buying a 1" headset replacement that fits older 27" tire road bikes but doesn't fit their bike.

Great. Thanks guys!