The two most useful readily available tools for electric flyers are a
Tachometer
The tachometer is used to measure propeller RPM allowing it to be compared with the expected RPM achieved by multiplying the motor kV (revs per volt) by battery voltage.
Watt Meter.
The watt meter measures the power being consumed.
Watts = Volts x Amps. Therefore knowing the watts and the volts the Amps can be derived.
Battery Checker
This devise will enable you to check each cell voltage at the field to ensure balancing is retained and dependant on the charge whether another flight could be attempted.
LiPo Charger and Balancer
It is imperative that all LiPo batteries are charged using a dedicated LiPo charger. Some chargers have in-built balancers if yours does not then it is necessary to purchase and use a separate balancer to balance the battery cells during charging. An unbalanced battery has a very short lifespan.