Most ESCs will slow down when your lipo hits a certain voltage. You will notice that the throttle does less.
Lipo alarms are great when you can hear them. They tend to be loud so you can fly around a bit but you have to learn the range.
Telemetry reports back to your ground station or remote. My Taranis tells me in English when I am running low on voltage. You can also put a voltage telemetry monitor on an OSD if you fly FPV. Voltage and current can be presented right in the HUD.
Timers also work well. You have to learn how long to set the timer for. My Taranis alerts me when I have been flying for x amount of time per model. I have one model that alerts at 3 minutes (4" Alien quad) and another that alerts at 19 minutes (Tiny Trainer).
Of all the solutions, the alarm
@LitterBug linked to is the simplest and my first choice for a new pilot.