12.8The accidental tells you where the harmony is going

This becomes a powerful reading skill.

Suppose you're in C major and suddenly see:

C♯

Maybe you're approaching D.

For example:

A7 → Dm

C♯ wants:

C♯ → D

Suppose you see G♯:

likely:

E7 → Am

G♯ wants:

A

Suppose you see F♯:

possibly:

D7 → G

F♯ wants:

G

So when a chromatic accidental appears, ask:

What note is this trying to resolve to?

That often reveals the secondary dominant immediately.