Hi Katarzyna

You can stain sperm nuclei using chromomycin A3 to evaluate protamine level in sperms. We have shown that it clearly correlate with DNA damage assessed using comet assay. The published paper is Alizadeh Mozdarani 2009, Reproductive BioMedicine Online;Vol 18 No 4. 479-485.

The relationship between DNA damage and direct quantitative measures of protamine levels in human sperm. Protamine concentrations (P1, P2, and total protamine) inversely correlate with the DNA fragmentation index. Additionally, patients with low P1/P2 ratios have markedly increased DNA damage. Previous studies have attempted to correlate DNA damage with indirect measurements of protamine levels using the chromomycin A3 (CMA3) fluorochrome

If you need a simple qualitative method you can try with aniline blue stain 

Depending on the equipment you have, the Aniline Blue stain is good and non expensive method to detect them.

 Dear Katarzyna Pluta

I hope this article will help you 

Chromomycin A3-staining as an indicator of protamine deficiency and fertilization

http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2605.1996.tb00429.x/abstract;jsessionid=542B8F264049E98C5D1E3092942477EF.f02t04.

Dear Katarzyna Pluta

if you need a simple method you can use aniline blue stain but if you want a specific method, CMA3 is really  specific and sensitive.