Most likely you're looking at a co-linear antenna. Lower frequency antennas are longer, higher gain antenna are longer. So you might have a low frequency and low gain antenna or a high frequency high gain antenna.
An 18 ft antenna at VHF 152 MHz is typically about 6 dB gain, at UHF 460 MHz about 9 dB. A 5 to 6 ft antenna at 2.4 Gig and 5.8 Gig could be anywhere from 10 to 15 dB.
antennas are related to their frequency, which in turn is related to their physical wavelength.
Just like grade school science - slap your hand in water = freq, and the wave crests are the wavelength.... faster slap freq = shorter waves.
Given that - a shorter antenna is used for higher freqs, and longer antenna is for lower freqs. You can stack antennas inside a fiberglass pole to produce gain, but that gain is omni all around the pole - like a donut.
Other antennas use a reflector - a dish - like a flashlight - to focus.
Sometimes the entire tower is the actual antenna, like with AM radio stations, very low freq compared to CB, 2-way, Wifi
780khz AM vs 101.9mhz FM vs 27mhz CB, vs 150mhz/450mhz/900mhz 2-way or 800mhz/900mhz/1900mhz for cellular phones, and Wifi using same freq as your microwave oven at 2.4Ghz
SO... not sure what your real question is -
A fiberglass pole you happen to see could, as was mentioned, be a single antenna for a low freq - like CB radios - or a gain antenna like for police, fire, WiFi etc.
If there are no markings, the only way to tell is to crack one open and look inside. If it's 5-6ft, it's probably a CB or marine antenna. The chrome bottom is most unusual and is not common among commercial antennas. My guess(tm) is some consumer application like a cordless phone extender (46/49mhz), CB, marine, or something similar.
Another way is to use an antenna analyzer.
The problem here is that this one only covers 1.8 - 170 MHz and
415 - 470 MHz. If it's 2.4GHz, cellular, WiMax, or whatever, it's not going to help.
For the antenna to be resonant and a decent impedance match to your wireless router it is going to have to be an odd multiple of 1/4 wl long. Since you say the antenna you are interested in has a metal tip I assume it is adjustable witha screw that moves in and out a couple of inches. If you have the proper test equipment it should not be difficult to adjusttthe antenna to the proper length. The antenna will have a considerable amout of gain in the direction of the axis of the antenna. Radiation perpendicular to the axis will be pretty much nil. IF you have the test equipment to tune it up and the radiation pattern is what you want then go for it.