Question

How can we differentiate between mini computer and mainframe computer

Answer 1

Main difference between them is physical size and at the time processing power, mini computer think PDP11 about as large as a washing machine, freezer. Main frame think really big computer it could easily take up the space of a smaller room, lots of storage, power usage, often redundant in hardware.
 
Mini computer and main frames has been replaced by PCs and virtualization. You get the same high availability as a main frame with several PCs running virtualization software in a cluster together with redundant networking and SAN storage. So today you can get more processing power from virtualized PCs than expensive proprietary main frames at a cheaper prize.

Feature wise in the past main frame computers was way ahead of PCs, as they had virtualization, redundant file systems etc.