1. Does it come from a traditional community?
2. Does it have variants (variations of the same song?)
3. Does it contain folk stylistic elements (melisma, certain scales or modes, manner of presentation?)
4. Has it been around for a while?
5. Is it taught aurally and passed down through generations?

Some songs written are in a folk style without being trad. You could argue that
these songs sound as "folk" as trad ones such as "The Shoals of Herring" by Ewan McColl. Or "This Land is Your Land" by Woody Guthrie.

Trad songs are just as valid as written or contemporary ones as they are still part of a bardic tradition.