From an SQL database implementation perspective, in my toy Python barebones SQL database that barely supports inner joins (https://github.com/samsquire/hash-db) I tested by testing on postgresql and seeing if my query with two joins produces the same results.

I ought to produce unit tests that prove that tuples from each join operation produces the correct dataset. I've only ever tested with 3 join operations in one query.

For a user perspective, I guess you could write some tooling that loads example data into a database and does an incremental join with each part of the join statement added.