How can I tell what level an enemy sentry is without entering its line of fire?

Commonly when roaming, I hear the familiar beeping sound of a sentry. Sometimes I know exactly where it is (such as around a corner right where it was destroyed last time), sometimes not. Is it possible to tell what kind of sentry it is without potentially getting myself injured or killed? It's helpful to make a judgment call on if I'm charging into battle against a mini-sentry or a fully-upgraded level 3. It's also helpful to know this information without giving the engineer a warning that I'm around the corner by poking my head out and having the sentry make that loud targeting noise.

Surprisingly enough, not only can you tell the difference between a mini and regular sentry, you can tell exactly what level the sentry is and how far away it is by its beeping noises:

• Mini-sentry: An above-average-pitched, single beep that repeats faster
• Level 1: A normal-pitched, single beep
• Level 2: A normal-pitched, double beep
• Level 3: A normal-pitched, triple beep

When you enter a sentry's line of sight nearby (such as around a corner), they emit a loud, high-pitched triple beep before firing. This sound is the same across all 3 levels. Mini-sentries have an even higher-pitched loud triple beep.

When you enter a sentry's line of sight from far away, the loud triple beep ascends in pitch. This applies to both sentry types in proportion to their respective pitches.