For plant patent applications filed after June 8, 1995, the term of a U.S. plant patent is 20 years from the date of the patent application. Older plant patents still in effect on June 8, 1995 have a term of 17 years from issuance or 20 years from application, whichever is longer. The U.S. Patent Office website has detailed information. Searches for plant patents may be conducted on the U.S. Patent Office website. Online search engines also find plant patents. To see if a plant is patented, try using Google's patent search. Search for the "varietal denomination," and for the name of the hybridizer (who is the Inventor). In the case of roses, the varietal denomination is usually a breeder code name. Some patent websites include information about patent applications. Plant patents are usually issued about two years after the patent application is complete. Even if you cannot find a patent using a search engine, for a recently released plant, check the website of the introducer of the plant. Plants that are in the patent process often show the notation PPAF, which means "Plant patent applied for."

Patent abstracts supply detailed and useful information about the botanical characteristics of a rose as well as its breeding. Even if the patent has expired, the patent abstract remains relevant to confirm the name of the hybridizer, the parentage, and the description of the rose. The The Rose: an Encyclopedia of North American Roses, Rosarians, and Rose Lore, by Sean McCann, p. 120:] 'New Dawn' was the first patented rose... 'Milestone' was the 5,000th plant to be registered in 1983... Most modern growers patent new varieties, but some now trademark the names instead, receiving a form of legal protection that can remain in force indefinitely.
[From the June/July 1999 issue of Garden Design magazine, which has several articles/notes of interest to rose growers. One of these is an article by Stephen Scanniello, formerly curator of the Brooklyn Botanic Garden’s Cranford Rose Garden, about roses Josephine grew at Malmaison and that are still available today, p. 32:] America leads the world in legal protection of new varieties. Plant patent laws created in 1930 give the finder or breeder absolute control of how a plant is marketed. (Britain didn't introduce such legislation until 1964; Canada did so only months ago.)...
[From Roses of America, by Stephen Scanniello and Tania Bayard, pp. 20-21:] Congress passed the Plant Patent act, providing much-needed protection for hybridizers. Up until that time, any plant that went on the market became common property, and the breeder lost all rights to it. The Plant Patent Act granted to anyone who discovered or hybridized a new plant the exclusive right to determine for seventeen years who would propagate and sell it. The first plant in the world to be patented under this law was an American climbing rose, 'New Dawn', introduced by Henry Dreer in 1930.
To find an online source for information about US Patents, see Organizations, Intellectual Property Network.