I have a few 18650s from laptop batteries.
I have put them one by one into a really cheap single 18650 phone charger.
('DIY Charger Box Case For 18650 Battery' on ebay)
I charged them up in the charger then discharged them by charging up my phone via a 'USB charger doctor' (again, on ebay).
Of course the figure that the charger doctor gives is at 5v having been boosted at something like 80% efficiency, but for my purposes, it is just fine as I can compare cells this way.
To get the real value for a battery I would guess you need to multiply by 1.4, but this really is only a guess.
The actual capacity of a battery will depend on start and end voltage of a discharge cycle - the further you go the more capacity you get but you'll shorten the life of the battery. The cheapo phone charger I use for testing goes from 3.05v to 4.15v, but you could go as low as 2.0v with some batteries if you dare though I suspect this remark will get replies with fire warnings. I have repeatedly gone down to 2.4v but then charge them back up in a fire proof area just in case. No flames ... yet.
Another parameter to check for is self discharge, which old and/or cheap batteries suffer from.
I fully charge the batteries, leave them to stand for an hour, measure the voltage with a meter, then measure them again a week later. If self-discharge would get them below 3v within in a year (multiply by 52), I won't trust them! (charge, internal short ... fire)