How do they prove intent to sell?

There's usually no direct evidence, so the State assembles an inference from parts.

Quantity relative to what a user would hold. Individual packaging — multiple small baggies rather than one larger quantity. Scales. Cash, especially in small denominations. Ledgers or notes. Phone messages arranging transactions. Sometimes an officer testifying as an expert that the combination is consistent with distribution.

Each piece is attackable, and the attack is usually specific rather than general. Packaging is how the substance was sold *to* the person. Scales are used by buyers who don't want to be shorted. Cash has a source that can be documented. Messages get read out of context or attributed to the wrong person on a shared device.

The phone evidence deserves particular attention, because getting into it required legal authority — and whether that authority existed is a separate question from what it contained.