"by" works the best. "on" also sounds natural to me. Here are some good sentences: Don't judge me by how I look now. Don't judge me on how I look now. Don't judge me by what I look like now. (I would most likely speak this sentence) Don't judge me on what I look like now. Don't judge me based on how I look now. Don't judge me based on what I look like now.

Is this an example from a book? The sentences are all kind of wrong ... but not because of the prepositions. "What" should really be "how." Then the sentences make sense! Don't judge me by how I look. Don't judge me for how I look. Don't judge me from what I look like. Don't judge me on how I look. "From" is perhaps the most awkward of the prepositions, at least in this example. But in truth they all work!

The translator says "by"but I don´t know if that´s the correct way. I have the same problem with prepositions.