Buy a Voltimeter for 10 bucks on Amazon. It’s likely 16v though.
AstroAI Digital Multimeter with Ohm Volt Amp and Diode Voltage Tester Meter https://www.amazon.com/dp/B01ISAMUA6/ref=cm_sw_r_cp_api_ZjDpAbZQ36PN5
That being said, even with a good high VA transformer, I’ve had issues with my Ring Pro and the chime in my house. Essentially when the Ring triggers the chime, the voltage drop is too much for the Ring and the video craps out. After a new power kit from Ring and two different transformers I relented and got the Ring Chime.