I disagree with your down vote because:
1. Loot changes if we kill the forgemaster, which can make us think that probably the ID of the boss we kill is different, probably same sprite just different ID. (Like all monsters that look the same) Also if we kill the forgemaster the boss won't hill so here we have 2 things affected by the forgemaster. Is there more? Who knows.
2. There are new bosses that bosstiary got affected with mechanich, big example "Rootkraken" if you killed it in his capsuled form, you didnt get the bosstiary (Tibia fixed it later because players noticed it and now you get the bosstiary both ways)
3. "By checking the bosstiary" I wanna think you mean, check the bosstiary looking for the "black shadow" of the Forgemaster? This could work if you have all or at least a lot of bosses discovered, but if you not, you will find yourself looking around 30 pages of black shadows (undiscovered bosses).