Calculate diode voltage drop
If all you can see is a diode and a resistor, with no other information, then you can not find the current through the diode.
You must look at the entire circuit. Substitute an ideal voltage source for a forward-biased diode and calculate the current. Use whatever exponential model you like to calculate the actual forward voltage of the diode at that specific current level. Change your ideal voltage source voltage to the calculated diode voltage. Repeat until the values of diode voltage and current converge to your satisfaction.
Or, run a SPICE simulation.