I have been watching Hocus Pocus and wondered how people in the 1800s knew when to add eth on the end of words.
The Authorised (King James) Version of the Bible would be a good resource for the old usage, since it is easily obtainable. The basic paradigm of the present tense in the old system is as follows. You will note that -eth is merely the third person singular ending of a regular verb:
1st pers. sing. I am, I have, I eat, I speak
2nd pers. sing. thou art, thou hast, thou eatest, thou speakest
3rd pers. sing. he/she/it is, he/she/it hath, he/she/it eateth, he/she/it speaketh
1st pers. pl. we are, we have, we eat, we speak
2nd pers. pl. ye are, ye have, ye eat, ye speak
3rd pers. pl. they are, they have, they eat, they speak
You will commonly see such spellings as "saist" and "saith" for "sayest" and "sayeth."
Note that at this point in the development of English, "you" had not yet replaced "ye" as the nominative-case form, nor had it yet replaced "thou" as the singular.
Note also that you will generally see locutions along the lines of "thou didst establish" in favor of forms such as "thou establishedst." The latter, while perfectly correct, was too awkward even for such dedicated archaists as the translators of the King James Version.
I'm pretty sure that '-(e)th' was largely obsolete by the 1800s. It was an alternative to '-(e)s' in conjugating verbs. The infinitive 'to run' becomes 'He runs.' in Contemporary English but could also become 'He runneth.' in Early Modern English depending on the dialect of the speaker or which they preferred in the situation. Over time '-(e)s' became preferred over '-(e)th' in almost all dialects.
Early Modern English also variably used '-en', '-(e)th' and '-(e)s' the same way we use '-(e)s' to mark nouns as plural, which gradually shifted to just '-(e)s' as it did for conjugating verbs, with just a few irregular exceptions ('oxen', 'children')
Of course actual speakers of EME didn't think about rules any more than a contemporary speaker does. How did they know when to use '-(e)st', '-(e)th', or '-(e)s' The same way a contemporary speaker knows it's 'I run.' but 'He runs.' (with a few dialectal variations) That's just what sounds right.
I haven't seen the movie but the non-historical movies I have seen usually have zero idea about the grammar of past times. They tend to add the '-eth' ending randomly just to make the language seem dated.
I could ask you, "How do you know to say, 'I am', 'You are', He is' instead of 'I is', 'you am' and 'he are'? The answer is that you know the grammar of your own language.
If you have learned French, German or Spanish for example you will know that verbs must be conjugated. The ending of the verb depends on the pronoun. In older versions of English this was true also. In modern English it only applies to the third person singular (he/she/it).
Compare the verb 'to go' in Spanish and in English of the older variety.
The further you go back in the history of English the more complicated it gets.
What happened to English is that various waves of invasion of England by people of different nationality and different languages gradually destroyed the grammatical structures. At one point, the ruling classes in English all spoke French. Later on educated English academics spoke in Latin when they were discussing academic subjects and in English for ordinary everyday purposes. Modern English is said by many to be a form of pidgin where the word order and context are more important than the strict endings of nouns and verbs.
We lost the ability to speak 'correctly'. If you want to know how to use the old verb endings correctly then I suggest you read about Old English and Middle English.
Also the Bible and Shakespeare are easily available sources of text that are entirely searchable online. They show by example how to use the correct endings.