How to find a burnt resistor value without schematic and manual?

take a multi meter . put the probes at the ends of resistor. if the multimeter shows infinite resistance that it means that the resistor is burnt out.

Actually it is possible if you have long experience in electronics.
If you can identify the function of the circuit or type of circuit the burnt resistor is connected, then you can apply a resistor ranging value safely.
Wheatstone bridge cannot be apply on mix circuits in such case.
For example if you have a burnt resistor at the output of power supply, you need to know the wattage of that resistor first then you can replace it with50K or100K potentiometer depending on the wattage of the burnt resistor.
Actually you can apply this method in many type of circuits.

It all depends upon your experience. The value of the burnt resistor can be had by precisely observing the circuitry and the function of that resistor.