How do I know when my razor blade is blunt?

Depending on how thick (coarse) your hair is, you should get 7 to 10 shaves from a Double Edge blade.

You will know when it is blunt (dull) as it will begin to tug at the hair and feel uncomfortable.  Once this happens you will know that it is time to change your blade.

Also read the Getting the most from your razor and blade article.