sprite_get_width

With this function you can find the width of the base resource sprite, with no transforms, in pixels.

Syntax:

sprite_get_width(index);

Returns

Real

Example:

if (sprite_width != sprite_get_width(sprite_index))
{
    image_xscale = 1;
}

The above code checks the width of the sprite as it is in the current instance and if there is a difference between that and the original base sprite, it resets the x axis scale.