"Oh, you really shouldn't have," almost always means you really shouldn't have.  It's up to you to figure out why.

"Fantastic" is a pretty good indication that you probably make an appropriate choice, unless the recipient is a sports announcer or writes ad copy for a living.  Then all excessive adjectives and superlatives have to be discounted.

"This is marvelous" probably means that the recipient really does think it's marvelous, unless the first syllable is drawn out, as in...  "This is mahhvelous, dahling," in which case the recipient is a community theater actress and intends to exchange your gift for something purple.

"Excellent" from a teenager used to be a high rating, but by now, who can tell?

Give yourself good marks for an unsolicited "Wow" spoken by any child under 10.  If the child says, "Thank you very much, it's just what I wanted," he's been coached.  Although your gift may be just what his parents wanted you to buy for him, it isn't just what he wanted and you should make a note never to buy the kid anything like that again.

"Lovely" from an adult female means she may like the present but has three of them in her dresser drawer already. "How nice," or "Very nice, thank you dear," indicates that you really blew it and she's wondering why in the world you would buy her such a dumb present.

If your gift is greeted with "How interesting" or "What a unique gift," you've strayed too far into the esoteric.  He hasn't the faintest idea what it is and you're going to have to figure out some tactful way of explaining it without embarrassing him by acknowledging that you know he doesn't know what it is.

"Well!" and "Now, that's a present" are reserved for items so incredibly ugly, inappropriate or ludicrous that even the best-bred are hard pressed to come up with anything more positive to say.  If your gift gets one of these, you're probably beyond hope.  Maybe you should make a New Year's resolution to just give everybody cash next year.