1. How I know that the operating voltage of small dc motor, where there is nothing written on.

2. why two 10nf capacitors are attached in searies with small dc motor and one 62 ohm resistor also.

If you know the speed or rpms, you could increase voltage until speed is reached, then measure voltage.
Is resistor in series with power and capacitors across motor terminals?

The RC networks are called snubbers and they reduce the emission of brush noise. They have to at the motor to be effective.

It is quite difficult to determine the voltage. I'm sure you can find some answers on the web. Wire size places a limitation, so does physical size. Take, for instance a starter motor and a bilge pump motor for a boat. Very different.

Determining short circuit current or the resistance when the armature is stopped will also help you, because that is a current you will not be able to exceeed.

Raise the voltage slowly while monitoring current. Pay attention to how close you are to short circuit current.

Monitor temperature as well. An excessive rise in temperature, is a sure bet that the voltage has been exceeded.