Is that the test point in a 2 wire plug front of strut tower driver side? If mixture is being correctly adjusted that test point at idle will alternate between 12V and 0V. If on all the time mixture is rich if off then mixture is lean. Any meter or an LED hook up can be used to monitor it. It is not strictly an 02 test point because other parts of the fuel system must be good to get a correct output here but that is what Volvo call it. I use that test point to adjust mixture.
A better way to test O2 sensor is to measure it's output directly with a digital meter. The connnector for that is on firewall passenger side. Voltage should swing randomly and quickly between .1 and .9 at this point.