At some point even the most experienced musicians come across music notation symbols that they do not recognize off hand. Whether the strange marking is found in a symphony or a piece of percussion music, note symbols and other markings all have a specific meaning. To read them you just have to know where to go look to decipher them.
By far the best and most reliable place to find the meanings of almost every music notation symbol ever written is to get ahold of one of the many different music dictionaries available today at your local music store or online through retailers like Amazon. There are literally dozens of these out there, but some are easier to navigate and find things in than others. One of the better visually oriented music dictionaries is the Essential Dictionary of Music Notation by Tim Gerou and Linda Lusk. The original book was so well received it also appears to have been reprinted by the large music publishing house, Alfred and is also available under their brand as well.
For those looking for more web based guide to music notation symbols an excellent, and free, web site to look at is Dolmetsch Online. This well established music education site has a long and solid history of teaching people to play the recorder but their online music education tutorials and information is top notch as well. Their Dolmetsch Musical Symbols page shows a large variety of images of musical symbols and other music notation elements, each one descibed or at the very least named for further research.
Of course, I would be remiss if I did not point out good old Wikipedia's rather extensive image based list of music notation symbols and other markings. Their list is quite detailed, and provides additional information about each marking aside from just providing the name of it as several other guides do.
Although all of the resources mentioned above are useful, there are still times when you will see a musical marking that you cannot easily discover the name of. In these cases it may be necessary to do something a little out of the ordinary. My experience has been that the newer, more robust music notation software programs such as Finale, Sibelius, MuseScore, and Maestro Composer all have a rather extensive list of markings and music notation symbols built into their programs. If all else fails download a copy of one of these free music notation programs or a trial copy of one of the commercial products and search through their menus until you find the correct musical symbols. Then, if need be use the dictionary or another source to find out a definition of what it does or how to play it.