> How can I tell more or less accurately with a simple method?
titration of a diluted sample - get a burette and a graduated flask.
formic acid is usually titrated against KMnO4, but if you can't get
that, then titration against a strong base will work too.
> can I just boil it to get to the 85%
no - bp of formic acid is 100.7°C! you could have found that out yourself,
lazy bastard!