Of all these bootleg sets everyone is talking about, did any of them not have a blue/purple recording surface? That has always been the telltale sign of a burnt (DVD-R) disc in the past. I'm pretty sure most (if not all) bootlegs would be tellable that way.
Most of the bootlegs these days of sets are actually "pressed" disks with accurate depictions of disk labels as well. Generally, the only way to tell now is based on the disks being single-layer instead of dual-layer and the packaging materials are typically of cheaper quality materials with possibly some blurriness to the artwork. Otherwise, they look legit.