Well, as you can see, my white leghorn roo is only a few weeks old, and he has been crowing for 6 days. I would think that your WL/RIR crosses would have started crowing by now as well. I guess you could just keep an eye out for pointed hackle and saddle feathers...
Here is a pic of my white leghorn roo that is only 3.5 WEEKS old.
If they are actually 3-4 months old, @drumstick diva may very well be correct. Please exuse my first comment as I paid no attention to the ages of the chicks' ages.