Cubemap.format
var format: TextureFormat;
Description

The format of the pixel data in the texture (Read Only).

Use this to determine the format of the texture.
	// Print the format of a given cubemap.
	var c : Cubemap;
	if (c != null)
		Debug.Log(c.format);
	else
		Debug.Log("No cubemap was assigned, please assing one on the inspector.");