If you want to know the mAH left, you'd probably need a battery analyzer. These do not come cheap - frankly it would be more affordable to simply replace the whole bunch of batteries at one go, unless you have a need for an analyzer/charger.
Analyzers can recharge batteries, so typically what you'll do is that you start it in charge mode, top up the battery, then let it run its discharge test. It will put a load on the battery and run it down, while counting the mAH. Once the battery is discharged to its minimum safe level (overdischarging DOES damage them), the analyzer stops and reports the mAH it successfully discharged from the battery.
I have used the Maha C777Plus2 and the Hyperion EOS5i AC/DC and they do the job well, they are not cheap though. I bought them as a charger for the varied kinds of rechargeables I work with.
If you want to know the mAH left without discharging the battery, guesstimation is the best way to go unless your battery has the electronics to record how much has been drained from it. Much easier to just put it back on the charger and top it up full.