Hi, I would not use plain text to confirm the existence of 'matched' records across multiple instances as this may result in errors\unexpected results and would suggest that if you require any record to exist across multiple instances, that you use xml to insert 1 record into any target instance that needs to match.
Regarding a REST query for searching on a text match; you would simply use any encoded query that returns the results you require, and I would recommend you start by testing using rest API explorer in your source instance to create your REST message\query.
