A man known only as "John Doe" to the authorities who have been investigating him since November was found unable to understand check forgery charges against him in Montgomery County yesterday and returned to a Maryland state hospital for continued mental evaluation.

"Doe" has been at Clifton T. Perkins hospital in Jessup. Md., since December while police in Montgomery County, the District of Columbia and Charles County, Md., continued to investigate his alleged use of dozens of stolen identification papers and credit cards.

In Mongtomery County Circuit Court yesterday, Judge Josephy M. Mathias said it was clear that the man was unable to assist in his own defense.

The judge accepted as evidence a psythiatric evaluation written Jan. 31 by two Perkins psychiatrists that says that "Doe" was "a schizophrenic, probably paronoid type" who posed a "potential danger to himself."

The evaluation by Drs. Jay A. Le-Bow and Stuart Silver said that during an interview "Doe" "appeared (to be a sad-faced youth who communicated in a halting stuttering fashion . . . and) is unable to understand the proceeding against him."

The report said "Doe" has an IQ of 64 and is a "mild mental retard."

"Doe" was in court yesterday accused of giving a Montgomery County woman a forged $650 check last September as payment for a car she was selling. Police said the check he allegedly used had been stolen several days earlier from a District of Columbia resident.

"Doe" will stand trial on the charges only if the psychiatric evaluation determines that he was sane at the time of the alleged act and is able to stand trial, Beck said.

The man also faces check forgery and car theft charges in Charles County, where he was arrested last year after allegedly stealing a car from man who refused to sell it to him. At the time, "Doe" was found to have several sets of identification papers. Because he refused to disclose his real name, his bail in Charles County was set at an extraordinary $1 million.