How can I know if an eyelid lift is what I need?

Blepharoplasty. Your best bet would be to have your eyes evaluated by a board certified facial plastic surgeon. There are many techniques used for different purposes.
Depends on problem. The eyelid lift is one tool amoung many that surgeons use to correct eyelid problems, bith functional and cosmetic. A consultation with a qualified, board certified, profession in your area can review the problem and select the most appropriate treatment.
See your doctor. See a facial plastic surgeon, ophthalmologist, or plastic surgeon for evaluation.
What do you need? If you want to look younger and have excess skin on the eyelids removed, then you need an eyelid lift or blepharoplasty. If periferal vision obstruction is the problem, then an appropriate visual field test can determine that and depending on the cause either excess skin is removed and/or the muscle inside the eyelid is tightened. Consult your ophthalmologist for the right diagnosis.
Obstructed vision. There are a variety of reasons to get an eyelid lift. The most common medical indication is due to visual field obstruction which can be confirmed with a special test.
Eyelid lift. It depends on what you feel you need an eyelid lift (or blepharoplasty) for. In some cases, it can serve to remove extra skin that is blocking vision, and in others is is a purely cosmetic operation. There are many different options - see a board-certified plastic/facial plastic/oculoplastic surgeon in your area to discuss your options.
Cosmetic or function. Depends on the issue: if the brow position is low, then the answer is really a forehead lift. If the problem is skin excess, the an eyelid lift (blepharoplasty) is the answer. If the issue is a droopy eyelid, then the answer is a ptosis repair. An evaluation by a plastic surgeon or oculo-plastic surgeon can help you sort this out.