Perhaps the following may help:
As a good first - and in many cases sufficient - approximation you can assume that the step response of the dc motor (number of revolutions over time) follows a standard second order system with two time constants T1 and T2.
If you measure this step response you can identify both time constants roughly by using the asymptotic lines crossing the time axis (T1) and the maximum speed line (T2), respectively. Then T1<T2.
Thus, you have the denominator N(s)=(1+sT1)(1+sT2) of the transfer function. The numerator is determined by the motor constant K (number of revolutions per volt).
This gives you the motor transfer function in the s-domain expressed by motor speed H(s)=K/N(s).
To transform this transfer function into the domain of the revolution angle you simply have to multiply by 1/s (integration in the frequency domain).