Böhm's theorem says that given lambda terms $r$ and $s$ with non-equivalent normal forms, there exists $\vec{a}$ terms such that $r\vec{a}=t$ and $s\vec{a}=f$.
I'm finding it hard to determine what those $\vec{a}$ are though. Even separating simple terms like $i$ and $k$.
Is there a procedure one can apply to figure them out?