Still looking for a unique birthday gift...
The first and most obvious question, is what does the person like? This would go hand in hand with, how well do you know the person? These two are incredibly important because they will determine your ability to shop for the person without help from another person. Enlisting the help of another when shopping for a birthday present is a bad idea because they say two can only keep a secret if one of them is dead. If you have to do it though, then you have to do it. Before you do resort to asking for help though, perhaps it would be better to employ an alternative method.
Without resorting to guerrilla tactics, you should keep an eye on the person and figure out exactly what it is they like. If you can't figure that out then it would be a good idea to watch things that they avoid. By doing this you can make a list of facts about them to determine exactly what it is they might like.
If you are shopping for a girlfriend or another woman in general, then you probably know that women like jewelry. If you are going this route then you probably know that there's not going to be an inexpensive route to take, so be ready for that. It's also a great way to create a personalized gift as it's easy to have a ring inscribed. If you know that there is a specific style of clothing that they like, then you can do your best to go clothes shopping for her, though if you are going to do this then you should probably actually take her with you and let her know what you're doing. There are a lot of people that do this, though the birthday surprise is ruined if you actually do.
Birthday gifts for men are usually a little easier because the majority of men love to receive tools, fishing equipment, or in some cases, sports gear. You could also take the easy route of video games or movies. Video games are a great choice because not only are they cool, they are the hobbies of some, which make it easy to come up with good birthday gift ideas.
If you're getting someone a present for their 18th birthday, it's actually pretty easy because you all you need to do is think of all the things that you couldn't have when you were a minor and then apply that to a birthday present. This doesn't mean you go out and buy them a heavy firearm, but you get the idea.
Finding the perfect birthday present may be slightly difficult but it will definitely be worth it in the end to see the thrilled look on the person's face when they get exactly what they want. It'll be even more fun watching them try to figure out how you knew. Though it may be a challenge, give it your best shot because you'll happy with the results!