Starting in 1961 the FBI and an army of other non-believers, wiretapped MLK. All found nothing but gossip. No one was able to nail him with any evidence of sedition.
Today, the laws on wiretapping have tightened up. Fortunately. The FCC regulations, two party consent rules, and political sensitivity have made this intrusive act practically extinct. So, don't try it.
On the other hand, if you want to know how to eavesdrop on someone far away...and without the same level of legal risk, we recommend you study this moderately low-tech video. Armed with this tutorial, a laser, a photocell, a battery, and a sound recorder, you can listen to conversations by recording the vibrations that bounce off a window pane.
This is a fun technology. The ethics of its application...well, that is up to you.