The best way to test it is with a real load, ie. in the mower while mowing a lawn. To do this you need a wattmeter that measures Ah, eg. RC Watt Meter DC 60V/100A Power Analyzer...
...and some way to insert it into the battery circuit. The simplest way is to wire it into the mower, perhaps using high current battery plugs such as these XT60 Connectors.
Then you can see how much battery capacity is required to mow your lawn. Record the Ah used in each mowing session until the battery runs out, and then you have its usable capacity (which will almost certainly be considerably less than the rated capacity).
Though you could build your own wattmeter, commercial units like the one above are so cheap that it probably isn't worth the hassle. As well as recording capacity they also show the peak current draw, minimum voltage, maximum power and watt-hours used (the most important figure).
I recently replaced the 24 V SLA battery in my mower with a 6 cell Lipo, and used this method to find out how much energy my mower uses. The information it provided gave me a lot more confidence than the 3 'idiot' lights on my old battery.