Most self-taught learners have the opposite problem from classroom learners: nobody ever tells them what is wrong. The risk is not that you make mistakes. It is that you repeat the same ones for two years until they are permanent.
Record, then listen as a critic
Speak for two minutes, then listen back with a pen. You will catch things you could never notice while producing them, because speaking and monitoring compete for the same attention. Do not try to fix everything you hear. Write down the two errors that repeat, and ignore the rest of the list entirely.
Compare your sentence against a real one
Take something you said and find how a native actually says it — in a subtitle, a book, a search of the phrase in quotes. The difference between your version and theirs is your correction. This works best on whole phrases. Single words rarely reveal the mistake; the combination does.
Chase errors that change the meaning
Worth fixing: wrong word order that confuses who did what, tense errors that move an event to the wrong time, a preposition that reverses a relationship. Not worth fixing yet: gender on nouns that context makes obvious, a missing article, a slightly odd but comprehensible adjective. These correct themselves with exposure and cost nothing meanwhile.
Keep a list of five, not fifty
A short list gets looked at. Write your five current errors somewhere you will see them, and check only those for a fortnight. When one stops appearing, replace it. Five at a time is slow and it is the only version people actually complete.
You cannot correct what you never hear. Record yourself, compare against real language, and fix the few errors that actually cost you meaning.