Testing the capacity of a lithium LiFePO4 battery involves fully charging the battery and then discharging it at a specific rate. Here are the steps you can follow:

1. Fully Charge the Battery: Charge the lithium LiFePO4 battery up to 100%.

2. Discharge the Battery: Discharge the LiFePO4 battery at a 0.2C discharge rate, which is 20% of the total capacity of the lithium LiFePO4 battery. For example, if the Lithium LiFePO4 battery is 100 Ah, you have to discharge it at 20 Amps per hour.

3. Record the Time: Record the time from the start of discharging to when the battery power cuts off.

4. Calculate the Battery Capacity: The capacity can be calculated using the below formula:
Capacity = Equipment Operating Current * Operating Time.

Alternatively, you can use a dummy load capacity tester for a more accurate measurement. This device will record the battery capacity automatically.

Please remember that these are general guidelines and may not apply to all situations or types of batteries. Always refer to your manufacturer’s instructions and safety guidelines when testing any kind of battery. If you’re not comfortable doing this yourself, consider seeking help from a professional.