[the_ivan]

There is a report titled Entwicklung und Flugeigenschaftsmässige Erprobung der Me 109 F (27.1.41) that briefly describes, as tittle says, the development and testing of the flight qualities of the F series, which are compared with those of the E series.
It's noted in the report that testing began in 1938. To be more precise, they tested diferent wings (slatless, wooden wings with ~4º wing twist) on Bf 109 B-1 W.Nr. 1032.