Access current mip level at fragment shader

Check out this old blog post from Aras.

This is the technique still used in Unity today for visualizing how much you are under/over sampling mip-levels. Instead of using a texture whose mips are shades of blue and red with varying alpha, you could instead make your own mip-helper texture— with different greyscale values from black to white in it's mip levels. If you sample that texture with trilinear filtering, you'll have a smooth scalar value of the current mip level.

Note that Texture2D.SetPixels takes a mip-level as a parameter and could be useful for generating your mip-helper texture.