The sonic test will tell the story. I've got a 390 FT block that can easily be bored to 428 size because the cylinder walls are really thick. A run of the mill 390 FE block won't be that thick, but some people bore those to 428 size too, although that is a dangerous practice. So the block you have will be revealed by a good sonic test. If you start seeing wall thicknesses less than .125", it is probably a normal 390 block that has been bored an excessive amount.
FT blocks also have a larger hole where the bottom of the distributor shaft goes in, because the FT distributors use a larger diameter shaft at the bottom (below the gear). Ford sold a bushing for the FT blocks that presses in the hole, and allows an FE distributor to fit. Does your block have that bushing pressed in? Or maybe a better question, has it ever been run with an FE distributor?