How can I find temperature of microcontroller?
At least some (I don't know about all of them) AVRs have a temperature sensor on-chip, which is connected to the ADC.


  The temperature measurement is based on an on-chip temperature sensor that is coupled to a single ended ADC channel. The sensor is a diode that produces a temperature dependent voltage. This voltage is measured with the ADC. The voltage has a linear relationship to temperature and the result has approximately a 1 LSB/°C correlation to temperature.  
  
  (from this application note)


A number of PIC devices also have an internal temperature indicator, like the PIC16F15xx and 16F18xx.


  This family of devices is equipped with a temperature
  circuit designed to measure the operating temperature
  of the silicon die. The circuit’s range of operating
  temperature falls between -40°C and +85°C. The
  output is a voltage that is proportional to the device
  temperature. The output of the temperature indicator is
  internally connected to the device ADC.
  The circuit may be used as a temperature threshold
  detector or a more accurate temperature indicator,
  depending on the level of calibration performed. A one-point
  calibration allows the circuit to indicate a
  temperature closely surrounding that point. A two-point
  calibration allows the circuit to sense the entire range
  of temperature more accurately.    
  
  (from this datasheet)




Further reading
AVR122: Calibration of the AVR's internal temperature reference, Atmel application note,
Using the AVR internal temperature sensor,
PIC16F15xx datasheet, p.133
AN1333: Use and Calibration of the Internal Temperature Indicator, Microchip application note,