You should say, e.g.: "Would you like to smell these flowers?". However, you can also use "sniff" in the context of flowers: "She gave the flowers a quick sniff to check they were real." In other words, smelling can be done passively, you are just breathing normally and a scent enters your nose. To sniff is to breathe in sharply through your nose, usually when trying to actively smell something. If you need further help with learning English, I encourage you to explore my teacher profile and please do not hesitate to contact me.

Sniffing can have overtones of disapproval or disdain. I wouldn't use sniff for anything fragrant! How about inhale? Imbibe is possible too, although it literally implies 'drink'