You could compare the square root to the floor of the square root and check if they're equal:

public static boolean hasCompleteSqrt(double d) {
    double sqrt = Math.sqrt(d);
    return sqrt == Math.floor(sqrt);
}

Suppose data is {3,2,7}. data.length is 3. This will compute sqrt((9+4+49)/3).

double sum=0;
// sum is now zero

for(int i=0; i<data.length; i++)
// Execute the following statement with i having each value starting from 0,
// incrementing by 1 each time (i++), as long as i remains less than 3.
    sum+= data[i]*data[i];
    // The sum+= statement is executed three times, with i each of 0, 1, and 2.
    // The first time adds 9 to sum getting 9
    // The second time adds 4 to sum getting 13
    // The third time adds 49 to sum, getting 62

double qmean = Math.sqrt(sum/data.length);
// make qmean equal to sqrt(62/3).

System.out.println(qmean);// Displays the final result.