Generally, a Jew knows if their origins are Ashkenazic or Sephardic.
One may even know if they come specifically from Lithuanian, Hassidic, Moroccan, Tunisian or any other community but they might also not, or they might not be sure.
In any case, a lot of Baalei Teshuva aren't certain enough about their Jewish heritage to know for certain which traditions were kept and what Halacha was followed by their family.
In this type of circumstances, how is one to establish which Halacha to follow?
Can one just choose the more lenient opinion in this case?
I'm using Shema as an example but this can apply to a lot of other things.
My Jewish ancestors were in Western Ukraine between late 18th century and the Russian revolution and I don't know where they were before then. My understanding is that MGA is Polish and that GRA is Lithuanian so I am guessing I should be following MGA.
Update: my question was closed with references to two questions that discuss the broader issue of choosing Ashkenazi vs Sephardic customs. I noted this right at the beginning of my question and intended to ask specifically about differences within the Ashkenazi community itself.