How can I know when to instantiate skidmarks in a car game

I'm assuming you have some variable measuring speed yes? At least for the skidding you could say something like:

 function GetMouseDown()
 {
    break=true;
 }
 
 function GetMouseUp()
 {
    break=false;
 }
 
 function Update()
 {
    if(speed > *a certain speed* && brake==true)
    {
       *Being creating the skid marks*
    }
 }

As for determining skidding if you've turned a certain degree at a certain speed it would be similar to what I just wrote except you would want to add in a rotation measurement.

so in the Update function you could monitor the change in rotation of your car...so something like:

 function Update()
 {
    if(startRotation-currentRotation >= *whatever number*)
    {
       *Begin creating skidmarks*
    }
 }

And at some point you'll also have to update the startRotation value so it doesn't skid on you constantly because you turned a corner or something from the start of the map.

Hope that at least gets you going in the right direction! -Kaze-