You would have to have access to the communications
between your ex-employer(s) and any prospective new
employers. If you asked . . . do you think they would
tell you? If you filed a suit you could get the information
through "discovery." However you really just can't go
around filing lawsuits because you "just" think two of
these bosses "might" be "blacklisting" you.
THIS ANSWER IS PURELY FOR ACADEMIC DISCUSSION ONLY AND DOES NOT CONSTITUTE ANY TYPE OF LEGAL ADVICE OR LEGAL REPRESENTATION.