India gets 7 happiness for every city it founds as opposed to 4 for normal Civs. Happiness for normal cities (not the capital) equalizes at size 6. However, this is actually not the full picture.
For every two pop over 6, India gets 1 extra happiness. In addition, India always gets a 1 every 2 size bonus in the capital, or 1/4 advantage compared to other Civs if both have Monarchy.
What does this all mean?
Well, this essentially means that an India founding a 4th city when its cities are 10, 8, and 6 is at happiness parity with a normal Civ. This sort of city size should be trvial to get quickly, and desirable for any Civ. Past this point, India should always be at a happiness advantage, unless you spam a ridiculous number of trash cities for some reason (don't do that).
At core city sizes 20, 16, and 16, India should be at +13 happiness relative to normal Civs, allowing happiness parity for 4 additional cities, assuming they are all size 1.
So. The way to leverage this is to grow, grow, grow your cities, and space them wide, with an emphasis on gaining tiles to work.
Once your happiness is booming, it can be advantageous for India to actually war to gain more lands and more large cities. In that sense, it's a warlike civ for the latter part of the game.