The Immaculate Conception is not found in nor supported by Scripture, the ECFs are in conflict over this. All the Roman Catholic is left with is "reason".
The Assumption is found no where in Scripture and again the ECFs are in disagreement over this.
You've yet to address my primary question.....how do you as a lay member of Roman Catholicism have the ability to determine what is or is not legitimate teaching?
And, on a lay person individual level, I have answered your question: use Scripture, under the guidance of tradition and reason. It’s a case-by-case basis.
Again, you need to give a specific example of a papal decree asking people to commit a mortal sin, if you want more specifics than the above. (Say what you will about them, but believing in the Assumption and the Immaculate Conception certainly does not detract from our state of grace or salvation.)