The Gist
Most reds are either acid-driven or tannin-driven — Nebbiolo is both, more than anything else. If a wine grips your gums and pierces your cheeks at the same time, the case for Nebbiolo is essentially closed. Tar and dried rose on the nose confirm it.
Mechanism
Most high-tannin reds (Cab S, Syrah) have medium-to-medium-plus acid because warmth moderates acid retention. Most high-acid reds (Pinot Noir, Sangiovese) have medium-to-medium-plus tannin because thin skins extract less. Nebbiolo is structurally anomalous: its thick skins provide very high tannin, while its naturally high tartaric acid content delivers very high acid simultaneously.
On the palate: if both tannin AND acid are simultaneously very high in a pale-to-medium red, the case for Nebbiolo is almost closed. Confirm with tar + dried rose on the nose. Nothing else on the the exam list achieves this structural combination.
Deeper mechanism
Sangiovese is the closest structural neighbor: both high acid and high tannin. But Sangiovese's acid is slightly lower than Nebbiolo's, and its tannin is angular rather than massive. The qualitative difference: Nebbiolo tannin is gripping and drying across the entire palate; Sangiovese tannin is angular and gripping specifically at the front of the mouth.
Confusion analysis
Nebbiolo vs. Sangiovese (structural)
Both: high acid, high tannin, Italian, savory. Quantitative: Nebbiolo acid and tannin are both higher. Qualitative: Nebbiolo tannin is massive/gripping; Sangiovese tannin is angular/front-of-mouth. Aromatic: tar+rose (Nebbiolo) vs. iron+sour cherry (Sangiovese).
Related varietals
This concept comes up when tasting: Nebbiolo, Sangiovese