How can you estimate evaporation from cooling tower?

Given the volume of the bassin is known, close make up valve and record the level dropped per hour.
Convert to volume dropped per hour and this will be the volume of water evaporating per hour.

i think, simply installed a water flow meter in fresh water make-up line of c/t and note the hourly reading. the difference b/w two readings, will be quantity of evaporated water.

am i right?

Not necessarily.
What happens to the blow down and drift losses then? U will end up measuring total loss of water by the above method unless you deduct the other two components!