If its a post 1972, it'll have a HIN on the top right exterior of the transom as GA said. If theres no #, then its likely pre-1972, or the HIN has been removed. Any # it has is likely a Serial# that wont be very helpful and not likely to ID the year, unless you happen to have the factory production records for Glasply.
Look under the dash, near the helm and on around the transom on the inside for an ID tag that might have the Ser# on it.
Those records have likely been lost to time a very long time ago.
Montana doesnt title boats or the seller just doesnt have 1? Thats an important question to answer 1st.
It looks like a run of the mill late 60s thru late 70s closed bow runabout
If the motor doesnt run, then its worth about the same amount as the trailer would be alone in Montana.
And probably needs a full rehab.