I have worked a lot with data migration and there is no such tool and do not expect one anytime soon, because the transformations can be very complex.
About the practices:
First you should test all your scripts.
Very that all the records has been migrated according to what is expected in all tables.
Verify that each transformation is correct comparing source and target data.
For numeric fields you can verify totals. Min and max values
For character field you can verify total lengths, Counts by lengths.
That is just to name a few. Other quality controls depend on business rules.