string cleanBsnNr = bsnNr.Trim().Replace(".", "");

// A BSN consists of 9 characters ...
if(cleanBsnNr.Length != 9) return false;

// ... all being numeric and not resulting in a 0 when converted to a number ...
long l;
if(!long.TryParse(cleanBsnNr, out l)) return false;
else if(l == 0) return false;

// ... the number must be validatable to the so-called 11-proof ...
long total = 0;
for(int i = 1; i <= 9; i++)
{
    // 11-proof voor BSN's: (9*A + 8*B + 7*C + 6*D + 5*E + 4*F + 3*G + 2*H + (-1*I)) % 11 == 0
    int number = Convert.ToInt32(cleanBsnNr[i - 1].ToString());
    int multiplier = 10 - i;
    if(i == 9) multiplier = -1 * multiplier;

    total += number * multiplier;
}

// ... not result in a 0 when dividing by 11 ...
if(total == 0) return false;

// ... and not have a modulo when dividing by 11.
return total % 11 == 0;

Good luck when you should build one for yourself. Checking your own built check can be done using this website with generated BSN’s (or your own, of course) by Menno Wilmans.