けり has 2 meanings: 過去 and 詠嘆. Is there some sort of trick to find out which one is being used? (besides context, of course)
- 詠嘆のけり is placed at the end of the main clause.
- 詠嘆のけり takes the 終止形 form unless 係り結び is involved.
- 詠嘆のけり appears more often in quotes than in narrative parts.
- なりけり is always 詠嘆.
- In haiku/waka, けり is almost always 詠嘆.
By the way, I feel 詠嘆のけり is somewhat close to 発見のタ in modern Japanese.