Telling hoarfrost from rime
Both coat surfaces in white crystal. They form by different routes and they look different once you know what to check.
Hoarfrost is deposition: water vapour going directly to solid on a surface colder than the frost point. It needs calm air and a clear night that lets surfaces radiate heat away. The crystals are faceted, often feathery, and they grow outward in the direction that had the best vapour supply, which in still air means roughly evenly. Hoarfrost is fragile enough to fall off if you breathe hard near it.
Rime is accretion: supercooled liquid droplets in fog or cloud striking a surface and freezing on contact. It needs wind and liquid water, so it forms on the windward side and builds into the wind, producing the horizontal spikes seen on summit instruments. Rime is milky and granular rather than faceted, because each droplet freezes too fast to organise into a crystal.
The quick field test is direction and texture. Even coating with visible crystal faces means hoarfrost. One-sided, into-the-wind, opaque and granular means rime. Mixed conditions produce both on the same object, usually with rime underneath and hoar on top.