I can generally tell the difference by looking at their faces.
I can usually see the difference just by looking at their faces.
1) I can generally tell the difference by looking at their faces.
「顔を見たらだいたい違いがわかる。」
generally で「だいたい」
2) I can usually see the difference just by looking at their faces.
「普通は顔を見るだけで違いがわかる。」
ご参考になれば幸いです!