How do we know that some pattern, rhythm, or meter was present in some of the Psalms? Many psalms were sung.
The first line of the Psalm states the pattern.
Many musical words and notes are used in the psalms.
Rhythm is detected when a psalm is read aloud.

Read this excerpt from we’ve got a job: the 1963 children’s march. as king finished speaking, a young white man named roy james jumped onto the stage and attacked him with brass knuckles, slugging him in the back and jaw. rather than return the blows or even cover his face, king dropped his hands by his side and looked at his assailant. other ministers rushed to restrain james, but king stopped them. "don't touch him," he cried. "we have to pray for him." king put his arm around the man who had been attacking him. they talked quietly. the man, a member of the american nazi party, started crying and admitted that he had come there to prove that king was not nonviolent. the minister did not even press charges.