I like to use Bradford dye for measuring general protein content. Here is the kit I use:

https://www.bio-rad.com/featured/en/bradford-assay.html

Do u want extract total protein , use trizol kit

and for quantification use Bradford or Lowry method

Measure total protein content in leaves first by estimating the total nitrogen content by micro-Kjeldahl method. Nitrogen content is multiplied by sample specific factor to obtain protein content.

Most proteins contain 16 percent of nitrogen thus conversion factor is 6.25.

Other methods can be used like Biuret and Lowry .