When “watching tweets”, I’m wondering whether the trigger only runs when a new tweet has been send from the watched account (smart), or whether it just polls every x/t and returns the last 100 elements, leaving us to figure out which ones are new?
If so, what’s the recommended way of detecting new items?
I was able to answer this myself by looking at the “Logs” from the Source tab. Indeed, the trigger seems to only return new tweets ! 
That’s correct! Each source in Pipedream has a deduplication strategy.
Pipedream will only emit new events from your sources if a new event has been detected based on the dedupe strategy choosen for that source.
You can read more about them here:
That way you don’t have to handle deduplication yourself!