Godot Version
4.4 stable
Question
GetTree().Root.GetViewport().AnisotropicFilteringLevel = myVariableHere;
is this enough to change the anisotropic filtering level? Is there anything else I need to do and is there a good way to test if its working?
If you can’t tell the difference, you probably don’t need it. 
More seriously, a decent test for anisotropic filtering is to put a texture on the screen with something detailed baked into it, like fairly small point size text. If you look at that from a severe angle, AF should make it much clearer.