Your bike is a load tester.
As long as they still meet your needs (get you to your destination), I wouldn't worry too much. It becomes a concern when you are actually getting close to fully depleting the pack by the time you get to your destination, because the individual batteries don't discharge at the same rate. Meaning it's possible you could damage one by overdischarging while the others are still permitting you to limp along.
A load tester will come in handy though if you decide to try the used batteries from the alarm company. Because you can go through a box of them and figure out which ones have the highest remaining capacities.
A simple capacity test is to wire a 12v battery to an AC inverter, and run a load on the inverter as well as an AC analog clock. The inverter will shut down when voltage drops below some threshold, and the clock will stop, giving you a useful data point to compare against other batteries using the same test.