Do you run windshield defrost while flying in IMC? how do you check that is working?
Usually with water droplets/ice you can see the effect but with just mist you have no guarantee of a clear view when becoming visual, yesterday evening in SE UK was the ideal day to see some mist/fog inside cockpit 
Is that what’s called a “misted approach”?
(sorry I know it’s bad but I couldn’t resist)
Maybe an aborted take-off ;-)
Yes, before I come to IMC, turn on Windshield defrost. It is difficult to control, but there is no better variant than proactively switching on.
I got it on a glider first time, descending via a big hole from a wave, took ages to clear up and missed approach or defrost were not options back then but it helps if you stop breathing, for takeoffs you just leave canopy open untill last minute and open air vents
I used to turn on defrost in IMC but don’t appreciate the smell and want to sopt any trace of icing on windshelds before it gets to the wings
I have no clue of the physics when the thing happens, do you need cold/hot air? moisture inside/outside? close/open vents?
I don’t think the need for windshield heating is any different in IMC.
I suppose if flying in IMC (100% RH) then eventually the RH in the cockpit will increase but it will never reach the required 100% because the temperature inside is much higher than outside (assuming you have a working heater). You can never achieve 100% RH by merely venting in external air at 100% RH if the destination temperature is higher than the source temperature.
Yes venting outside air and humidity and letting air/windshield temperatures to equalize then you are right
I think it is much complicated than that you have two dew points inside/outside (relates to RH inside/outside) and two air temperatures inside/outside as well as windshield temperature, so you get all configurations: descend from VMC on top with a hot windshield, run into a patch of rain or just sitting on the ground…
In a car, it is uch easier you just run aircon & defrost if moisture condenses inside and windshield wipers if it condenses outside
If the windows are cold, and the inside air has high humidity, condensation will form on them.
Inside humidity is caused by water evaporation from warm, moist, large surface area, lungs, breathing out air warmer and more humid than you breathe in.
Good ventilation will help, regardless of temperature.
The better the ventilation, the greater difficulty in keeping cabin warm.
Blowing warm air to the windscreen will evaporate the condensation.
Avoiding breathing is not a desirable option.
On the topic, from a wave flyer
