Kind of a loaded question. It will depend on where you are, how close you are to an established BFE, and what kind of resources may be available to the engineer doing the analysis. Your best bet is to call a local Civil Engineer/Surveyor familiar with your region's topography and flood flow characteristics. Even if the first one is a miss, they'll probably have an idea of who to call.
Out of curiosity, why are you trying to establish one? FEMA has done a good job of updated most of the FIRMs in the past several years - and I'd be surprised if Louisiana had been left out. If you don't have an established BFE but you are in a region that does flood, your municipality or whatever governing body in charge of your building code enforcement likely has a floodplain development ordinance requiring a minimum elevation above existing grades when a BFE/DFE is not established.
