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.
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