Before firing, use the time that you are undetected to take note of your surroundings. Find all available targets and decide which to take out first. During sniper missions, you can use the thermal scanner to find human targets easier. However the time that the scanner can be used is limited, indicated by the gauge that is visible below it. The gauge will refill before the next mission. Consider zooming out fully first before turning on the thermal scanner in order to get the best view of hidden targets, then turn it off.