Active voice directly attaches a subject to an action, whereas passive voice makes this connection somewhat partial.
In the passive voice, the subject isn't at the start of a sentence. For example, 'The Guide to Wattpad was written by Sophie'. The object being 'The Guide to Wattpad', the verb or action being 'written' and the subject being 'Sophie'. Instead of this object-verb-subject sentence structure, which is longer and less clear for a reader, a subject-verb-object sentence structure is seen as more desirable. So: 'Sophie has written The Guide to Wattpad'.
Or 'The hole was dug by the dog' with the object ('hole')-verb ('dug')-subject ('dog') sentence. Improved in active voice as 'The dog dug the hole'.
For a reader, placing who's doing the action first is important to avoid 'roundabout sentence's' and maintain straightforward narrative that's easy to follow. For example, passive voice would be particularly confusing if you're writing about more than one character. Saying this, passive voice could be desirable if you want to put more emphasis on the verb or object than the subject experiencing them. However, the majority of your sentences should ideally be in active voice.
So, how do you definitively tell if a sentence of passive or active voice?
Obviously it depends somewhat on the structure of the sentence in question, but try to identify the subject. Once you've done this, you should be able to move it to the front of a sentence with relative ease.
You could also look at the many examples online, essentially helping you to learn and therefore recognise the uses of both voices in your own work.
Editing or re-editing your work once you're familiarised with some examples, you should be much easily identify where you tend to slip into passive voice.
At the end of the day, using passive voice unknowingly, I don't think is the worst writing mistake by a long stretch. Especially since, if you want to create some sort of disconnect between a character and their actions, passive voice could be used to good effect. However, learning the difference, which hopefully I've helped with here, is the best way to effectively identify, particularly passive sentence structures, in your own writing.