The GUI.Toggle function returns the state of the toggle. true = on, false = off

toggleState = GUI.Toggle(toggleRect, toggleState, "toggle");

I've found out the solution..here is the code I'm using

    toggleDayNightNew = GUI.Toggle (Rect(25, 20, 80, 30), toggleDayNight, "Day/Night");
    if(toggleDayNightNew != toggleDayNight)
    {
        if(toggleDayNightNew == true)
        {
            GameObject.Find("CrateCamera").GetComponent("toggleSky").applyDaySky();
        }
        else
        {
            GameObject.Find("CrateCamera").GetComponent("toggleSky").applyNightSky();
        }
        toggleDayNight = toggleDayNightNew;
    }

var dayMaterial:Material;
var nightMaterial:Material;
private var ButtonText:String ="day";
RenderSettings.skybox = dayMaterial;

function OnGUI () {
    if (GUI.Button (Rect (10,10,150,50), ButtonText)) {
            if (RenderSettings.skybox == dayMaterial){
                ButtonText = "night";
                RenderSettings.skybox= nightMaterial;
            }else if(RenderSettings.skybox == nightMaterial){
                ButtonText = "day";
                RenderSettings.skybox= dayMaterial;
            }
    }
}