What evidence is there in Act 3 Scene 6 that Lennox in convinced that Macbeth was responsible for the Murder of King Duncan?

Lennox's speech is filled with verbal irony.  Even though
he does not state his suspicions directly, it is clear that he is sarcastically blaming
Macbeth for the recent murders.  For example, he
says,

The
gracious Duncan

Was pittied of Macbeth; marry, he was
dead.

And the right-valiant Banquo walked too
late;

Whom you may say, if 't please  you, Fleance
killed,

For Fleance
fled.

By tying the two
murders together--Duncan's and Banquo's, Lennox is suggesting that anyone who gains
Macbeth's sympathies suddenly finds himself dead.  He preposterously places the blame
for Banquo's death on his son Fleance, just as Malcolm and Donalbain were blamed for
Duncan's death because they ran.  Just as Fleance's escape did not mean that he killed
his father, Malcolm and Donalbain's fleeing Scotland does not mean that they killed
Duncan.

He goes on to seemingly praise Macbeth for "nobly"
killing the guards because they would have most assuredly denied killing Duncan.  Of
course, they would have denied killing Duncan--they were innocent.   He then comments
that if Macbeth were able to capture Malcolm, Donalbain, and Fleance, they would all be
put to death for supposedly killing their fathers.

In this
fashion, Lennox shows that Macbeth is the culprit in the murders of Duncan and Banquo
and that he attempted to place suspicion on others who fled or whom he killed.   At the
end of this speech he calls Macbeth a "tyrant,"  and at the end of this scene, Lennox
hopes that Scotland will find relief from its suffering "under a hand
accursed."