How do you know when you need deicing?

[lehbird]

I was taking off at KRKS and while it was cold ~20F but there was no precipitation and I could not get off the ground. I barely reach rotation speed and that was it, would fail to climb and crash. Tried 3 times, same result. 

I also did not see any visual signs of icing. (Which I should, correct?)

So how do I know when I need it?

Thanks,

Dave 

[softreset]

This has been the recommendation on other threads.

Popup Icing Display - Utilities - X-Plane.Org Forum

[dizzyflores]

The popup icing display is a great utility. 

A good rule of thumb is deice when the temp is below 10c and anti-ice when the temp is below 10c and visible moisture is present.