"Any one have any idea how to detect it using normal 8051. "

8051 by itself is incapable of seeing threads !!

Yes, the method suggested by Leo is normally used in textile machinery - the wheel is coupled to a zero speed switch to set off a trip in case of thread break.

The other method is a micro switch coupled to a tension lever to the end of which is a idle roller and the thread passes on that - keeping it pressed at an angle. In case of a snap of thread, the lever springs back triggering an alarm.( And the lever must have a hysterisis or float to avoid nuisance triggering )

OK all these presuming that the thread has SOME tension in the line - and it should be so - there is no line that is "zero-tension".

Raghu