When you see people parking their Hyundai's and Honda's to travel by the local train :)
When you start travelling with an umbrella as soon as its the month of June :D and when you know that even if its pouring heavily you have to go to office/school/college
The last point is actually a complete contradiction to the way I have been brought up for the 18 yrs before the last 5 years.....
From where I come, if it rains.... you stay indoors till it stops raining. You call up to confirm if the school is closed...so basically when it rains... you get a day off and so you rejoice.... but here.... if it rains (which it does after every 5 min in those 3 months) you just promptly open your umbrella and go on with your work.... and the only pain about mumbai rains is the traffic jams...otherwise who wudnt want to experience what actually "raining" means :)