Personally, if I see a SEGA logo on the box, it's a SEGA game. However I also clarify things with a tier system:
1 - Internally developed and SEGA owned and published
2 - Externally developed and SEGA owned and published
3 - Externally developed and developer owned and SEGA published
Things can also get even more detailed when you factor in specifics like the exact point SEGA got involved (tiers 2&3) and how much internal SEGA staff assisted in development. Sonic, for example, is that first tier. Bayonetta is second tier (and the game itself was SEGA influenced and some SEGA staff assisted in development). Jack Lumber is third tier, as it was already developed and the dev owns the game, SEGA just published it.
Of course there are always going to be special cases, like ASRT and Sonic 4, where it was externally developed but it deals with a SEGA license. In that case it could be considered both tier 1 and tier 2 (1.5?).
Resonance of Fate would be a second tier title, same goes for Ecco and Toe Jam & Earl. Right?