Nowadays it is very hot in India.My PC shutdowns after random time.Reason is increased speed of CPU fan.

I would guess that the increased rpm of the CPU fan is not what causes your computer to shutdown, it's a symptom of the underlying problem (the CPU is overheating) as the fan controller on the board tries to keep up with the CPUs cooling demands.

If I am able to access the cpu fan speed,

You can probably get a tool from the manufacturer of the mainboard, or use one of the many free tools (e.g. Speedfan), to monitor the rpm of the fans in your computers. The same tools will also allow you to check the CPU temperature directly (at least with most modern processors).

However, I would try to address the main problem instead of trying to figure out when to shut down.

You could first try to clean the fan and the heatsink on the CPU (preferably with compressed air). These things collect lots of dust after a year or two of use, and a layer of dust acts as a very good insulator, keeping the heatsink from properly cooling the CPU.

You could also try to underclock the CPU and/or reduce the CPU core voltage (can be done with most CPUs to a certain degree, but you'd have to figure out how much reduction your CPU can handle).

You could also try to install a larger heatsink or larger/more powerful fan, but since that most likely means spending money, trying the first two options first is the cheaper thing to do.