How do conferences detect double submission? My first year undergraduate teammate has submitted our project work to multiple conferences. I want him to pull back the submission before reviewers take action against us. But he wants proof that conferences actually care about it and can detect self plagiarism and double submission.

This is in electronics, where conference proceedings count as publications.
"Actually care about it": Read the conference's rules.

"Can detect": Irrelevant.  Like many things in academic research, this is primarily honor system.  If your teammate thinks that whether something is acceptable is determined by whether you can get away with it, they need a serious attitude adjustment.

By the way: do you have a faculty advisor on this project?  They need to know about this.  They would also be the best one to give you advice on the specific norms of conferences and publishing in your fields.