The real experts (PhD type Fisheries Scientists in the LMB genetics field) say you can't tell one from the other on looks. It requires genetic testing. I have seen them both separate , together and in all mixes many times and agree that it can't be done safely on looks alone. There are some traits but it takes to long with them out of the water to id them. My suggestion is forget about trying until at least 2 years after they went in. That way the genetics will have been passed on to the next generations. Here are some pics but I don't think they will help.

LMB from Greg

Northern LMB







F-1 LMB








Flas (& RES)