How to determine an oxidation number of an atom in the compound, that has at least two elements, which are not hydrogen or oxygen?
In this case, the potassium ions are +1 each, and the oxygens are -2 as normal.  That leaves uranium in a highly oxidized +6 state.  The piece of information I think is missing is that Group 1 ions are almost always in the +1 state, because they so readily lose their outer valence electron.  A molecule that is not an ion, like your example, must have an overall charge of zero.