Updated To Craft 3, How To Delete Craft 2 Plugins
It partially depends on the plugin. The one guarantee you have is that there will be a row in the plugins table. Go ahead and delete that row.

Some plugins create 3rd party tables. For most of those plugins, it's probably safe to just delete those tables as well.

I know you can't go back in time, but... the best solution would have been to remove those plugins before updating. Virtually all Craft 2 plugins did a good job cleaning up after themselves in the database.



Based on your comment...


  "Form Builder 2, AmForms, Hacksaw, Shortener"



Form Builder 2 - Not very familiar with this, but looks like it has 5 tables. Probably safe to delete them.
AmForms - Another one with 5 tables. Probably safe to delete these as well.
Hacksaw - No DB tables
Shortener - No DB tables