Geological maps are the interpretation by a number of geologists and are generally the synthesis of extensive field work. The only place with the real evidence is the field (including geophysical work and drillholes, if available obviously). In many countries geological maps are accompanied with a guide discussing certain interpretations, especially regarding faults, which are quite often up for discussion. Maybe you can find the author of the map and ask them if such a guide is available, or even directly ask on what base the fault was drawn? I think you'd find many geologists willing to discuss their interpretation.
Other indications ("evidence" is a bit strong) for thrust faults can include an unexpected contact between two layers, a sudden change in orientation and/or thickness of the layer(s).
If you're really sceptical, try to sketch a small geological profile and see if you can make a different interpretation.