I concur with the post above that you would be better off to hire a battery maintenance company with the experience.
There's no simple kit answer that works...
If you really want to get into this, do some extensive reading e.g. here
I've read everything I could find on the subject and, the above papers seem to be the best collection of sound engineering on the subject, with the least amount of self interest bias among folks hawking impedance testers.
Battery Impedance Testers have been around for a long time, but the controversy on their ability to produce meaning results still lives on..
Personally, Alber is the only one I would even consider because it does a true DC load test versus some testers using AC impedance measuring techniques that grossly ignore the capacitor (or surface charge) effect.
Many of the AC impedance testers are based on baseline impedance values not published by the battery manufacturers, nor supported by the battery manufacturers.
We have talked to the engineering staff of several major industrial (stationary) battery manufacturers and the DC load tests are the only data they will accept on a warranty claim.
If you don't want to use something like an Alber, measurements at 6 month intervals of the individual cells looking for voltage deviations on a per cell basis can be done. But this method needs knowledge of fall back techniques to verify questionable looking cells (e.g. individual cell load tests, etc) and experienced judgement to make the right call whether the battery string will hold during an outage.
If you decide to do this on your own, you need to do a candid analysis of what is the price of failure is to you, and your organization, if you have an system outage due to battery failure...
