Good question.
One obvious but commonly overlooked way is to look for decals or control photos that call out auxiliary functions.
Otherwise you want to look for hydraulic couplers, extra lines, or pumps. Standard will be two couplers at the front of the arms, high flow will have four. Along with that comes additional lines. There may be an additional pump as well.
Remember, the presence of auxiliary hydraulics doesn't always mean high-flow. Standard-flow is common on most skid steers, while high-flow is often an optional upgrade. That's a mistake I've seen made before.