Either ask your travel agent or if booking at flybmi.com then look at Fare Summary on Page 3 of the booking process and it will tell you what the Fare Basis is.
It'll be a code like "HX1OWBMI" or "YX1BMI" - the first letter is the one that indicates how many miles you'll earn - in these example they are H fares and Y fares, earning 0.5 and 1.5 x base mileage respectively.