How to tell "health" of an 18V battery that won't hold charge.

I've seen somewhat similar posts, but couldn't find the same question...

*TL:DR* - How do I tell which one of my 4ah batteries is old and not holding charge?

I've been using 18v 4ah batteries to run my mower reliably for several years. Used to be able to mow whole yard on 1 - 2 batteries, now its 2 - 3+. So - are least one of my 4ah batteries is not holding charge. I bought a new 4ah HP battery to replace one of the old ones.

Is there a *relatively* easy way to tell how much "life" is left in the batteries, so I can "cull" the worst one from batch? (Cull = recycle)

Yes, I sharpened the mower blade first.