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This is a Tachinid indeed.
In general, it is very difficult to explain why.
Most Tachinids are more bristly than Calliphoridae, as Gerard correctly explained. But then again, most Phasiinae (subfamily in Tachinidae) are less bristly and often taken for Syrphidae - lookalikes !

Most males in Calliphoridae have the eyes very close together, leaving only a very narrow vertex.
Most Tachinidae have, in the male sex, a broader vertex.

This one clearly has a narrow vertex and therefore ... it is a Tachinid.
(Yes, I know, ...., but don't complain at me !)

The species is quite like Ernestia rudis, a large and quite common species. Its larva parasite Panolis flammea, and therefore it is confined to coniferous and mixed forests.
The males are often found occupying a territory from a leave of, say, Fagus.


Theo Zeegers