I do it by experience but usually Mucronella is always on conifers with more delicate or separate spines. In the microscope both genera are very similar. Macrochemical reactions are also good at least for Mycoacia. But there are just two Mycoacia species in Europe that resemble Mucronella in color, M. uda and M. aurea and I tell them apart by the naked eye. The other species are much stouter or differently colored or smelling. By the way, Mycoacia has been subsumed into Phlebia, at least the European or boreal species.
I have to admit this is really difficult to say.