You can't look up a license plate online for free, because the Department of Motor Vehicles, which is the government agency that issues license plates, is obligated by law to protect the privacy of every vehicle owner in its database. The agency does not sell or provide existing vehicle registration records to any public or private entity.
If you come across a website that claims to offer free instant license plate searches, you are likely looking at a false advertisement or a scam. Generally, these websites lure users in by saying that their service is free, but after you enter the number of the license plate you're searching for, instead of providing you with the information you need, you'll only be given confirmation of its availability and you will have to pay a fee to access it.
Even if information on the license plate number is actually available on the website, it probably comes from old and out-of-date databases that may yield useless information, because current information is protected.
If you want to look up a license plate number online, the best you can do is use trusted paid license search websites such as People Public Records, Search Quarry and Vehicle Data Registry. These websites may charge monthly, quarterly or bi-annually for their services, but you can trust the accuracy and reliability of the information they provide.