Get product being added to cart in event

[StijnCoolen]

Is there any way (using EVENT_POPULATE_LINE_ITEM) to only hook into the product that is currenctly being added to the cart.
I want to perform logic (check certain fields and show a warning) when someone adds a product to the cart.

EVENT_POPULATE_LINE_ITEM seems to run for every product in the cart when I add a new product to the cart.