Hi Kristijan, I go by watching the color of the red hot charcoal, But you cannot see that with your set up so will need a different plan. First of all, are you committed to using water vapor as an adjunct to the air? If so, I see no need for you to use engine exhaust. The water vapor will perform the same function as the exhaust gas in cooling down the oxidation zone as the water is reduced into H2 and CO. In this case you may run a problem of adding too much water which will condense out in the filters. If so, cut back on the amount used. I have also noted the humidity in the air can also affect the amount of water used. A cool humid day may have enough water vapor in it to preclude the use of injecting water vapor.
If you also want to use engine exhaust, the best way to monitor the oxidation temp is probably with a thermo couple. This is something I have not used yet and cannot speak much about. You will be dealing with temp of 1800F on the low end to about 3000F on the high end. I find only a little engine exhaust is effective, so another thought is to just use a little and see what happens. Whoops, maybe add a lot, and then cut back. If you add too much, you will cool the fire and make a lot of CO2 and not have much power. Then start cutting the engine exhaust back a little bit at a time.
My 2 cents for now, Gary from PA