Disclaimer - I'm answering this from the ELL perspective, so this answer says nothing about how the ELU community would like to handle these sorts of situations.
If you're aware that there is a question on ELL that might answer the question on ELU, I would suggest that you link it in a comment as was done for the example here. There are lots of questions on ELL that have links to relevant ELU questions even though it's rare that we migrate a question. I view this as similar to being able to find an answer elsewhere on the Internet. Just because there is something that you could read somewhere else that answers the question doesn't mean that there is no potential value in an answer from this site's community of experts.
If you feel that having both questions on ELL would be good because they're asking the same thing in different ways, go ahead and migrate it after you leave a link. I think the example here is a good candidate because it asks about "turn off" instead of "turn down". Even though the "turn off" question is answered in the "turn down" question, having the "turn off" question makes the answers easier to discover. If you don't think there's much value in that exact question being migrated over the ELL in light of the potential duplicate you've found, then the link and maybe a nudge to check out ELL is enough in my opinion.
If you just suspect there is a duplicate, but don't know for certain, go ahead and migrate it and we will sort it out. I think it goes without saying, don't migrate complete junk. If a question is off-topic here because it's proof-reading, or unclear, or too broad, it will be off-topic on ELL as well. If it's off-topic because the author is obviously looking for an ELL-style answer and not an ELU-style answer, or isn't fluent enough in English to meet the research standards on ELU (but has written to the best of their ability and asked an answerable question), send it on over.