Glad the immediate problem has been solved.
"So Is there a way of knowing whether my oil seal/o ring and gasket change has been successful before putting shoes and drums back on ? "
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If you failed to clean the surfaces thoroughly, you didn't remove old bits of gasket,the surfaces were badly damaged, you got loads of old oil/ grease and dirt on the new gasket,new O ring,etc, etc....you generally made a complete pigs ear of the installation ...then you... might... see some weepage.Changes are that nothing will show.
Once the oil heats up, when you drive the car, being thinner it will then find any available gaps and leak.
It's sounds to me ,from your general approach to maintainence, that you take care to get it right, so I don't anticipate there being a problem.
However, for your own piece of mind why not put it all back together, take it for a spin around the block to get the oil in the diff nicely warmed up.Put it back on axle stands again and whip off the drums to check for leaks.A can of brake cleaner/ spray bottle of meths, rags at the ready, should it be pouring out.My bet is that it will be dry as a bone !!
Bob