How can I interface with arduino mega2560 to detect the presence of a 48 V without pulling down the voltage by more than 0.1 V
I can spare in the order of a few Milli amps for measurement, as I do
  not want the 48v to drop
  
  I'm measuring 5V using an arduino. So input impedance of an analog pin
  is 100Mohm


To be able to tell how much your 48 V will drop due to this, you must provide the source impedance of your 48 V rail.

Non the less, if you can spare a few mA of current for the measurement and have 100 Mohm input impedance downstream, I would recommend the following:

This will consume just under 2 mA and give you about 20 % margin until you saturate your measurement.







simulate this circuit &ndash; Schematic created using CircuitLab

EDIT: With your updated information (for future reference here on EE.SE, please make sure not to sprinkle vital informaion in comments and include as much as you have in your original question), this above is my updated schematic. It will load your 48 V rail with about 45 uA, causing a &lt;1% voltage drop across your 10 kohm source impedance.