Flyback Converter Transformer Leakage Inductance
How do we use LT spice to find the desired leakage inductance. Is
  there any other way we could do this ?


The simple answer is that if you know the turns ratio you can "move" the leakage inductance from one side of a transformer to the other by multiplying or dividing (as appropriate) by the turns ratio squared. For example, if the primary has a leakage inductance of 100 uH and has a step down ratio of 10:1 then that leakage inductance on the secondary side is 100 uH divided by \$10^2\$ = 1 uH.

If all you know about the transformer are the primary and secondary inductances (and I don't mean leakage inductances) then you can arrive at the turns ratio by taking the square root of those inductances. For instance a transformer with primary inductance 49 uH and a secondary inductance 1 uH has a step-up or step down turns ratio of 7:1.