HC-SR04 ultrasonic sensor: How to check voltage on the ECHO pin I'm using and ESP8266 with HC-SR04 ultrasonic sensor. The sensor requires 5v input and will also output to the GPIO pins at 5V. 

ESP8266 has a strong recommendation to use 3.3V into the GPIO pins. I've found some setups online with people using voltage dividers to bring the ECHO pin voltage down. 

Example on a PI: https://tutorials-raspberrypi.com/raspberry-pi-ultrasonic-sensor-hc-sr04/

I'm using a basic multimeter that i've found to be pretty accurate. When I put the BLACK cord on the ECHO and RED on the GROUND (of sensor pins), I get variable voltage readings. I'm trying to ensure a 3.3V coming from the ECHO into my ESP.
Questions: 


Am I putting the black and red cords on the appropriate areas? 
If yes to question 1, then why am i get such a variable of readings?
How can I test to ensure only 3.3V coming out of ECHO?
You have your leads backwards. But that doesn't matter, since there's nothing to measure. You'll only see a voltage (and even then only a brief pulse) when an echo is actually received.

You would need an oscilloscope to be able to read that voltage.