|The universally accepted onsen mark|
Since most onsen are made by drilling into naturally hot underground spring water, many are located far away from town centers, usually up in the mountains. Also, many onsen are attached to hotels and therefore they only keep limited hours for non-over night patrons or simply refuse customers without reservations. Therefore, if you are hankering for a hot bath then its best to plan ahead and find out what onsen are open in your area. Hot springs that allow for drop-in visitors are called hi-gaeri onsen (日帰り温泉).
For those with basic Japanese skills, there are plenty of resources available to you. The mapping website, Mapple, offers a full onsen guide by region. Mapple's competitor, Rurubu.com, also offers a similar guide. Both guides point out amenities like roten buro (露天風呂 or open-air baths that usually offer a scenic view), dining facilities (shokuji shisetsu 食事施設), car parks (chushajo 駐車場) and so on.
If you are hip with the iPhone crowd, there is a nice free application called onsen sagashi (温泉さがし or "onsen search") where you can use the phone's built in GPS to locate info on near by springs.
Finally, many areas that are well know for their onsen may have their own multi-lingual guides available. For instance, Beppu in Oita is extremely famous for the quality and quantity of its springs and maintains a relatively useful English language website. Other areas such as Nagano and Niigata also have onsen tourism available in English.