JimB is right about the bench testing problem. For some background, here are some ways to do this.
An engineer would use an instrument called a Vector Network Analyzer. This is an instrument that sends some RF energy towards the antenna, and then measures exactly what voltage and current is reflected back. It then computes some useful things like real and imaginary parts of the impedance, the VSWR and/or the return loss. The frequency where the VSWR is lowest is often the frequency at which the antenna is designed to operate.
There are so-called "poor man's" vector network analyzers, available out there. They are often called "Antenna Analyzer" and can be relatively inexpensive as long as you only want to measure up to 300MHz or so. You can even make your own, but it is an advanced project.
One old-fashioned way to measure the frequency of an antenna is to use a "noise bridge" (google for more details). In fact, some antenna analyzers use the noise bridge principle instead of the VNA method.
Another method that I've used many years ago was to get my hands on a "gate dip meter". This is an oscillator with a calibrated frequency dial, and which allows some of the oscillator energy to radiate. A detector is included inside the meter which measures the oscillator energy. When you hold such a meter near a coil or antenna that has a strong resonance, that coil or antenna will suck power from the meter and cause the meter needle to "dip" (hence the name). In use, you hold it steady near a small coil attached to the antenna feedline, then vary the oscillator dial until the meter dips. Very handy little gadget.
In some cases, it is possible to simply attach the antenna to a receiver that tunes over a very wide range (like scanners do) and operate the receiver in AM or SSB mode across that range and just note where it tends to receive the most noise off the air. This often indicates the frequency at which the antenna is being effective.
As Jim mentions, someone with experience in antenna design can often guess the frequency of the antenna by inspecting it for key things like:
length of elements
feed location and method
relationship of one element to another
type of connector used
but of course the easiest way is to find a model number printed on the antenna and then just look it up.
good luck