The best option is to get a flight controller with a built in current sensor. But another easy thing to do that is not quite as accurate is to simply connect your battery to the Vbat pins and enable it within cleanflight/betaflight. Then if you are using the blackbox data-logging you can see the battery voltage when you takeoff and compare that to when you land to work out how many volts you use per minute of flying. Or you could just measure battery voltage via a voltmeter before and after takeoff.
Not the most accurate method but will give you a feel for how long you can fly.
Typically a wattmeter is used on the ground between your battery and quadcopter (as they are kind of big and bulky) then you can bolt your quadcopter down and see how much power/current your quad is using at various throttle settings.