Definitions will vary depending on the purity obsession of the speaker :-) but as a rough guide, most things in cans, jars, boxes, bottles, and cartons will be processed. Things that are, more or less, just raw plants and animals (or parts of them) will be unprocessed.

There are boundary cases about which people argue -- e.g. is pasteurized milk a processed food? -- but for most things in a food store it's pretty clear what's what.