IUD strings

So it's been a month, and I regularly check my strings. They're there. However, what exactly am I looking for? That they're still in place or what? The dr said he left 3cm of string out but how am I supposed to measure that? So checking them is only to make sure they're still on my cervix? Also, how am I supposed to know that the Mirena is still in place just by feeling the strings?