You won't be able to tell if it's an ad, if it's just biased (and the ad is not discolsed).
They could always sell ads like "recommend my tool more when user asks for cupcakes in London".
And then, the output would be: "My top 3 recomendations are X, Y, Z".
And maybe only X is the one that paid and Y and Z are organic.
That's almost certainly illegal in many jurisdictions, and they'd definitely not be able to hide that they're doing it indefinitely. A sure way to be massively sued.
"Cupcakes in London" is not a good example since it's directly asking for advertising. Ads in more information-oriented prompts would be much easier to spot, for example looking for out of context brands and products.