"It is easy to sense the fully charged condition in CC type charging. Just monitor the current going to the battery"

NO! - A CC (Constant Current) charger will, by definition, (attempt to) maintain a constant current through the battery irrespective of its state of charge!
If you just keep pushing more & more current into a fully-charged battery, you risk it exploding! (but see the 10% rule below)

"A 100A battery is fully charged when it is drawing less than 1A current"

NO. That depends entirely on your charging circuit - this cannot be taken as a blanket statement!

BTW: that's 100Ah (Amp-Hours) - let's not confuse the guy with sloppy notation!

"10% of the rated capacity is taken as standard charge rate."

This is often quoted as a safe rate, at which overcharging won't cause problems (explosions, etc)

"But modern batteries offer fast charging, where you can go upto 50% of the rated capacity."

Many you can charge even faster than that - but then it becomes absolutely imperative that you have proper endpoint detection and safety interlocks.
Typically, a fast-charger will have an ambient temperature sensor (as the battery characteristics are temperature-dependent - it is unsafe to fast-charge a cold battery), a dV/dt end-of-charge detector, a battery over-temperature cut-out, and a maximum charging time cut-out.