Trickle chargers are used to safely charge automotive and other batteries. The chargers have at least one meter as part of the controls. The meters can indicate the amount of amps flowing from the charger into the battery (ammeter) and voltage supplied by the battery (voltmeter). Use these meters to test the trickle charger.
Connect the trickle charger to the battery and monitor any gauges on the trickle charger. Look for identifying labels such as "Ammeter" and "Voltmeter."
Read the setting on the ammeter. There may be a gradated scale on the meter. This meter indicates that the trickle charger is sending power to the battery and that the trickle charger works. Indications such as "Charge" mean the trickle charger is sending low, safe amps to the battery. "Start" is found on combination trickle chargers/vehicle starters and indicates high amps are being sent. In any case, any movement in the ammeter needle from zero indicates the charger is working.
Look at the voltmeter. It will tell you how many volts are available in the battery. Movement of this needle over time indicates that the trickle charger is performing its job. For example, if the voltmeter shows 6v at the beginning of the day and 12v at the end, you know that charging has occurred. Voltmeters usually work whether the charger is switched on or off.