To test for "Brownout" you are testing for adequate current delivery to the Rx us that will maintain voltage.You have a fairly accurate system for that built into your radio via telemetry.Even if you dont use it inflight get a TM module and use it for setup at least.I power everything up and physically grab the servo arms and load the heck out of them against their holding power.Its a bit difficult with 4 servos and 2 hands though.It helps to get an extra set of hands when testing a setup. You should not have more than 1/2 volt of voltage drop from unloaded to loaded. 3/10 is better.You should never have voltage drop below 4.5
If your using a DSMX Rx or other with data port you will also have Flightlog data on your screen. This gives a much better idea of how your Rx/antennas are performing than a simple go-no go low power/range test.
Remember "brownout" is loss of voltage at the Rx bus.It is not signal related.If you lose power the Rx will reboot.During reboot you will lose Connection.If the loss of power is severe enough the time to reconnect may be more than a fraction of a second.When a system first connects it will take a bit of surge current(amps) for a split second as the servos jump to position.If the power delivery is marginal enough it may force another reboot.This can happen over and over and it may never reliably reconnect inflight.
To be honest if you want reliability and you use a BEC I leave the TM installed inflight and set an alarm for a tenth or 2 below what my avg minimum Rx voltage is.BECs dont generally go bad all of a sudden.They get wimpy and lose a few tenths of a volt here and there until they simply cant keep up and then shut off.The Rx voltage alarm has saved me more than one model.
Flightlog data inflight is the same kind of way.If you lose a remote or cable inflight I found that you will get a momentary Hold registered with no noticable loss of control long before it
develops into an extended Hold with loss of control.
If you log your data and you do have trouble you can look ack at the data graph and pretty much see exactly were the problem is without it being a total guess.Its either a power problem or signal problem.
I used an Eagletree logger before we had telemetry and it was good for setup and logging but I didnt get a realtime alarm when a problem was developing inflight.Had to plug a USB into it and look at the data on a laptop.Telemetry has made keeping tabs on the basic system health so simple.Still have to load the logfile on a PC to read it later but at the field I can glance at the mins and FL data at the end of a flight and see if it looks ok.
There is a bit of a learning curve to understand what it is telling you but once you are comfortable with it theres no going back to Plug and Pray.