Measuring a voltage that is higher than your reference voltage
Suppose you have an ADC with a reference equal to the supply voltage you're trying to measure (5.0V in this case). 

If you use an LM431 shunt 2.495V reference (that's a cheap one, there are ones with much tighter tolerances available) with a resistor you can calculate the supply voltage. Call the 2.495V input Vx 

Since (for a 10-bit ADC) ADC_count = 1024 * Vx/Vref, we can calculate 

Vref = 1024*Vx/ADC_Count. This will give a result with 9-bit resolution (accuracy will depend on how accurate your Vx is). 

So if Vx = 2.490 and your ADC_Count is 0x212, then the supply voltage/reference is 4.81V. 

If you don't like having to calculate the reciprocal, you can operate the ADC with a reference that is regulated and divide down the supply voltage so that is within the ADC range. This would also give more resolution if you don't divide it as much as 2:1. 

You can also connect the LM431 with two resistors to give you (say) a 4.0V nominal Vx, which will also improve the resolution. Or use another reference with a higher output voltage (4.096V references are available).