You can measure the voltage directly from your battery terminals with a digital multimeter.
Etekcity MSR-P600 Digital Multimeter / DMM / Multi Tester with Capacitance Test (Red) - - Amazon.com
Everyone should have one and know how to measure a couple things. Voltage being one of those. DC or AC voltage is easy to measure. One con is that this is not a permanently mounted solution that you can glance at but it is more accurate and more useful.
Also it has been mentioned that your tow vehicle will charge up your battery after a couple of hours. This is not really accurate. And while not all vehicles are the same I would guess that not many vehicles are even capable of fully
charging a trailer battery. The voltage at the trailer plug is not sufficient to fully charge and not sufficient to quickly charge a battery either.
Bulk charge voltage is usually around 14.4 volts and should be higher than that according to battery manufacturers (14.8v).
Trailer plugs are most likely around 13volts and will likely put out less than 10 amps.
So in a perfect world it would take 4 hours charging at 10 amps to restore 40Ah which would charge a group 24 battery from its 50% state.
The reality is that you may start charging at 10amps while your battery is low but the amps will taper off quickly as the battery voltage rises. So your tow vehicle can probably get you from 50-80% fairly quickly (~2.5 hours) but it will take days to get the rest of the way on what is essentially maintenance voltage for a battery of around 13.2 volts.
Hope that all makes sense and isn't too far off topic!