With Texel Density Checker you can: 

• Calculate Texel Density for model for different texture size (512px - 4096px) or with custom size and aspect ratio
• Rescale UV for getting texel density what you want
• Use Interactive Checker Material
• Copy TD from Active to Selected objects
• Vizualize Texel Density with Vertex Colors
• Vizualize UV Islands with Vertex Colors
• and more..

It is Addon with GNU GPL License. You can download source code from GitHub

Texel Density Checker simple for use. You need select your mesh (or faces) and texture size and just click button.