pulse transformer's have two important characteristics:
1) Its primary to secondary turns ratio
2) Its maximum primary volt-seconds.
Let me explain 2):
Assume that the transformer has a maximum spec of 50 V-μS.
If you are driving it with a 5 volt squarewave, its on period should be no larger than 10 μS, otherwise it will saturate.
If your maximum duty cycle is 50%, that means that the maximum total period will be 20 μS, or 50 Khz minimum operating frequency.
You could build a variable frequency power squarewave oscillator. Start with -let's say- 100Khz @ 5 volt, 50%. Monitor the output with a scope and slowly start lowering the frequency until you start seeing a droop in the output voltage.