Trying to work out the pinout of an RJ10 (or RJ 11) RS232 port,
Standard RS-232 uses both positive and negative voltages, usually in the ballpark of +/- 10V. Because generating high voltages and negative voltages requires some extra circuitry, a lot of devices today use the RS-232 signaling with standard logic levels instead, like the 0V/+3.3V of your ESP8266.

Since you're measuring +10V from your sensor, that suggests that it's standard RS-232 and will also output -10V. If you have a scope, watch it when it's sending data and confirm the two levels that it toggles between. It should be idling at the negative voltage, so it's a bit unusual to be measuring a positive voltage like that unless it's sending a lot of zeros.

If you're only receiving data from the sensor, you could probably get away with a voltage divider and diode to keep the signal between 0-3.3V, followed by an inverter (as the negative voltage corresponds to a logic 1 and positive is 0). If for some reason you end up needing to send data, you'll need an RS-232 transceiver to convert the 0-3.3V to the higher positive and negative voltages needed by standard RS-232.

Another option is to find the RS-232 transceiver chip on your sensor. If it's using 0-3.3V on the logic side, you could potentially remove the chip and connect the logic levels directly to your ESP if they're not too far away from each other.

One other thing to consider is that if the sensor has the ability to receive commands over that port, you should tie off its input to something to prevent the floating pin from triggering random input data. Ideally this would be to the negative voltage, but if that's not available you might have to figure something else out.

And one final suggestion to confirm the pinout and basic functionality: connect it to a USB-serial adapter on a computer, start up a terminal app, and make sure you can see the incoming data.