In the Radio Control model airplane hobby, a servo tester is a small handheld device that allows the user to manipulate a servo. You can use a pot to move the servo through its range of motion. Some have other features. But basically, it simulates using the RC transmitter and receiver. You plug the servo in and test its movement.
It is very easy to use an arduino and a pot to make a servo tester. I bet google will turn up many examples.
But mostly for you, you want a way to try out the esc arming.
Securely mount the motor on something so that if it runs unexpectedly it does no harm.
connect the ESC to the motor and your servo tester (or arduino with a pot).
connect the power to the esc.
Does the motor beep? That is one of the common steps in the arming sequence.
Turn the pot down so that you send a 0 degree (or 1000uS) command. Any beeps.
Turn up the pot a bit. Does the motor run?
If not, turn the pot all the way up. Any beeps?
Turn the pot back to zero. Any beeps?
For most of my ESCs that require arming, zero throttle for a few seconds, full throttle for a few seconds and zero throttle again arms it. After that, moving the throttle spins the motor.