How to detirmine if I need a gate driver for MOSFET or not?
You are likely using MOSFETs as hard-switches i.e. like relay contacts and the main thing to watch out for when driving the gates is the gate to source capacitance. It might be as high as 10nF and your driver might only be able to supply 30 mA (say).

This means that the rate of change of voltage applied to the gate is highly limited by both the gate capacitance and the lack of driving current to inject charge into the gate capacitor.

I = \$C\dfrac{dv}{dt}\$ i.e. how quickly does the gate voltage rise for a given injection of I

So dv/dt =30mA/10nF = 3 million volts per second.

This may sound quite high but, considering that you might need 10V to fully turn on the mosfet, you'll need to wait 3.33 us. Given that rise times and fall times ought not to be a significant proportion of the period of any PWM waveform (maybe 5% maximum) you find that the maximum PWM frequency ought not to be greater than (say) 7.5 kHz.

Rephrasing, 7.5 kHz has a period of 133.33 us and no more than one-twentieth of the time should be taken up in the device switching on and off thus, the time for switching on and off is limited to 6.67 us.

So if you need fast switching look into the MOSFET data sheet and do some calcs - you might find that a 1 amp driver is the only way to get certain applications to work efficiently.