If there are 5 induction motors running at a time,  how can we check the initial p.f , in order to install a capacitor bank ( if required)? Let all motors are 10HP.

there's power analyzer/ tester that can be used in analyzing the behavior of your loads, including power factor... viewing other significant parameters like harmonics, unbalances, and distortions... these factors are also prerequisites in installing such capacitor banks in order to improve the power factor of the system... if you ignore this and focused on the power factor alone, then you may get surprised when your capacitor banks may become instant fireworks (due to overheating) when energized due to harmonics...

If harmonics are not an issue, contact motor manufacturer for recommendations of a single motor and multiply by 5. You don't want to exceed this or you could overexcite the motors upon opening the disconnect that feeds all of them. But also in this regard, you must always operate all five all the time the cap bank is connected assuming you are applying a fixed bank and not a stepped bank.

Checking the overall power factor is easy, just attach your power analyzer on the feeder that supplies all five motors.

Where to place the capacitors is the challenge; if the motors are always going to operate together because of process requirements, place the capacitors in the same spot as the power analyzer. If the motors can be switched on and off independently then you will need to place the capacitors at, and sized for, each individual motor.

Do check to make sure that you don't create a potential resonance condition with the rest of the plant.