Well, if in doubt in the simulator use the temp on the PFD or what they call it on the ATIS. The temp displayed on the ground in the real airplane can be affected by things like direct sunshine and then you need to go with the ATIS.

In the sim they will tell you the temp and that should be what's displayed on the PFD. If the temp and visibility call for anti-ice then that's what you should use. If they say it's 4ºC and the visibility is 1/2 mile well, then you'll need your anti-ice for taxi and takeoff.

And if the paperwork field report says that there's snow on the runway or taxiways then anti-ice must be used accordingly.

It's attention to detail and application of limitations. I don't say that to be condescending but it's all part of how this job and training can have some pitfalls. Just pay attention to details like visibility and temp. Too often we are concentrated on other things in the sim and forget about these details.