Generally we can "see" shock waves when air conditions are right either to see the normal light refraction due to density variations or moisture condensation.
YouTube has plenty of clips which let us all see these sorts of phenomena.
For instance, in the
at around 1:50 we can see a number of shocks during a high speed pass, including at the back of the canopy discontinuity .. although one needs to look closely to see the latter. The SR71 model test shows a number of shocks along the aircraft's profile.
shows a spanwise shock over the top surface of a typical jet transport.
As to exactly where shocks initially form on a given aircraft will depend on the particular aircraft profile.
.. I make no comment about the dreadful background music which some seem convinced is mandatory for YouTube clips ...