How to decide whether or not to use isolation in a circuit that has a microcontroller and a 48V DC supply?
Your question is not totally clear, but I assume that your uc must measure the 48V, not just know whether it is (within some wide margin) present at all.

First: Is it a problem if the 48V and the uc share the ground? If so, you will need isolation and you will have an interesting design challenge.

If a common ground is no problem, I see no need for isolation (unless there are considerations that you did not tell us about).

You need an input voltage range that your uc can handle, with an impedance that is not too high for the A/D input. You can use a resistor divider: choose the lower resistor equal to the highest impedance that your A/D allows; choose the upper resistor to divide the maximum of your range-of-interest (for instance 30 .. 60V) down to what your A/D input can measure. 

If the 48V can be higher than the highest value you considered you can either take that value into account when calculating the upper resistor (which reduces your accuracy), or add some overvoltage protection on your A/D input (like a zener clamp, or skottky diode clamp to Vcc if it can handle the extra current).

If you or your safety guys are worried about the consequences of a failure of the upper resistor you can split it into two equal values, and design the clamp to handle the current when one of the two shorts.

Do note that the accuracy of your measurement will be lower for lower '48'V voltages, and it will depend on the accuracy of your reference voltage (often the Vcc of the uc).