How to make a decision without an if statement

Since you've used a for loop, here's a solution just using two for loops.

public static double pay (double salary, int hours) {

    double pay = 0;

    for (int i = 0; i < hours && i < 8; i++) {
        pay += salary;
    }
    for (int i = 8; i < hours; i++) {
        pay += (salary * 1.5);
    }

    return pay;
}

This sums the salary for the regular hours up to 8, and then sums the salary for the overtime hours, where the overtime hours are paid at 1.5 * salary.

If there are no overtime hours, the second for loop will not be entered and will have no effect.

To avoid direct use of flow control statements like if or while you can use Math.min and Math.max. For this particular problem using a loop would not be efficient either.

They may technically use an if statements or the equivalent, but so do a lot of your other standard library calls you already make:

public static double pay (double salary, int hours) {
     int hoursWorkedRegularTime = Math.min(8, hours);
     int hoursWorkedOverTime = Math.max(0, hours - 8);
     return (hoursWorkedRegularTime * salary) +
            (hoursWorkedOverTime  * (salary * 1.5));
}