How can I check the charging capacity of SLA batteries?

How can I check the charging capacity of the sealed lead acid batteries which I use to power my fish finder?  I have two battery boxes.  One of the batteries provides enough juice for two days of fishing.  On a recent trip, the other battery pooped out after nine hours of fishing, which brought my day to a premature close.  A salesperson at Batteries Plus told me that SLA batteries only last for 2-3 years, and they don't have a way to test the capacity of a battery, but his job is to sell batteries.  Any suggestions?  Or should I just go buy a new battery?

2-3 years? Depends on use and care. Yes, you can do a couple different kinds of tests on them. I have one of these:
https://www.amazon.com/BA5-100-1200-Cranking-Electronic-Battery/dp/B0017R5EQK/
 
It does everything as advertised. Works great! It will tell you a lot more than just a volt meter can- including capacity.

As for the Batteries plus guy suggesting SLA batteries have a certain lifetime, that's nuts.

SLA batteries like to be slowly discharged (like from a FF) and like to not go below 50%...

this data sheet (for a battery that I use on my Oasis) has a useful graph that shows "Life Characteristics in Cyclic Use" (bottom right on page2):

https://www.atbatt.com/amfilerating/file/download/file_id/789/

the deeper the discharge, the more of the battery life you 'destroy'...

here is the link for that battery at Atbatt.com:

https://www.atbatt.com/power-sonic-ps-1290-12v-9ah-sealed-lead-acid-battery.asp

another thing they 'like' is being recharged immediately after use (really really important after a deep discharge), this reverses the chemical reaction and minimizes the amount of 'life' you have spent.

cheers, roger