Shelf clouds really do not indicate storm severity at all, but rather are simply visible evidence of a storm's outflow. Indeed, up in Darwin or even south-east Queensland, shelf clouds seems to be as common as cockroaches given the very humid atmosphere.
That said, if you see a storm that has a smooth wedge-shaped shelf cloud that angles up into the storm's base, that to me suggests there is some substantial inflow happening above the outflow and, whether by 'force-feeding' or storm-scale processes, such a storm is more likely to be severe. Furthemore, if you can see a semi-circular shelf cloud or actually see evidence or a broadscale organised rotation, as well as simply outflow 'churning' and turbulence, then such a storm is quite likely a supercell and hence almost certainly severe.
Might be some examples of what I am talking about somewhere on the site here.
