The question is meaningless, since transmission range depends on (1) transmitter power, (2) transmit antenna, (3) channel, (4) receive antenna and (5) receiver sensitivity. Maximum communications range depends on all of these factors, so the 'range' of an antenna is totally meaningless.

Instead you want to think about the radiation pattern (gain over an isotropic radiator as a function of direction) for a given antenna. This is measured using a radiofrequency anechoic chamber in which you can mechanically move the antenna under test through angles theta and phi while it transmits a signal with a known power level, and receive the signal using a simple dipole antenna with well-known gain at the frequency of interest. You measure the received signal amplitude as the antenna under test is moved through each pair of (theta, phi), calculate the transmitting antenna gain and generate a polar plot of the gain as required in 2 or 3 dimensions.

If you don't have access to any of that, you might try building an accurate simulation of the antenna (e.g. in NEC). This can also give you estimates of antenna impedance and other interesting parameters as a function of frequency.

A "transmission range" is defined by case basis, using an uninterfered E field level (or power density or H field in some cases). For instance, in absence of interference, the level condition for FM stereo broadcast is E>43dBuV/m in rural areas and higher in urban areas. Even if you don't have environment contraints, you should define a threshold field level, which will be your marker for coverage area. In simple propagation path (without obstacles for instance) the transmission range could be the distance from tx when the E threshold condition is met. If the environment is complex, the coverage cannot be simply described as a "transmission range". Maybe you mean "antenna pattern"?

Deniel  has rightly said. The Range is relative term and dependant on the factors.

However you can measure strength of signal at a distance using "Field Strength Meter" within the sensitivity rof Meter.

I agree with Daniel.

I wish to only add that it is possible to use a vectorial network analyzer to automate the measurement of a radiation/reception pattern of an antenna. One can also measure the gain of an antenna by comparing it with that of an known antenna. However there are various method to measure the gain.

Best Regards

Thank you for all your advices & suggestions...Amipara i am using CC2420 radio which comes integrated Wireless Sensor network motes such as MIcaz, from which RSSI (Recieved SIgnal Strength Indicator) value can be measured by extracting RSSI value from packet metadata.. Suppose i am transmitting packets using CC2420 radio (operates @ 2.4GHz with data rate of 250kbps) @ power level of 0dBm in an area without obstacles (line-of-sight) scenario and if i know the gain of the antenna and receiver sensitivity is their any mathematical relation of finding the transmission range???Why am i asking this is bcoz XBee radio with U.FL antenna claims outdoor (line-of-sight) range of 90m and indoor range of 30m... If so,how exactly are they determining the transmission range?? Are they measuring transmission range manually or is their any mathematical way of doing it.??

Indoor transmissions are always suffers with lot of obstructions like walls, furnitures etc and their thicknesses and materials. All these  reduces the length as signal gets reflected, refracted, disperse. Thus heavily suffered signal will offer quite less range as compare to outdoor one. The range claimed is estimated range and genuine manufacturers defines surrounding conditions with range/signal strength at given distance. Estimated value is based on empirical results. 

The range si calculate from the propagation or radar equation. that means your range doesn't depend only on the antenna (the transmitter) but also the receiver and the propagation attenuation and losses

For free space (no walls or reflections) you can use the Friis equation: http://en.wikipedia.org/wiki/Friis_transmission_equation

Pr= sensitivity of the radio=-95dBm

(http://www.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=cc2420&fileType=pdf)

Pt=0dBm you say. Gr, Gt are known. Lambda is obviously known. From the equation you can calculate the maximal range. Obviously, this is not taking into account diffraction, absorption etc. These guys wrote an interesting document on the topic:

http://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1292&context=abe_eng_conf

In Broadcast engineering contour maps of same field strength is drawn by transmitting at constant power and depth of Modulation , then taking measurements with GPS assisted field strength meter and draw the graph on the map.