For an air-filled coaxial line, the half or a quarter-wave length is straightforward. For dielectric-filled coax cable, the propagation speed is lower, so the cable will be shorter. How shorter you can find from manufacturer's data.
If you cannot find the data, follow this procedure:
1. Take an approx. quarter-wave long coax cable to sacrifice by an experiment. Solder one end to make a good short (sheath to center conductor. ) Open the other end and separate sheath from the center conductor.
2. Take a RF signal generator with a coax output at the desired frequency (e.g.123 MHz). Connect a diode detector so that the live point is exposed. Connect an analog indicator (Vmeter. mA meter) to detector output.
Set the RF level to 1...10 mW, the detector output would be ~30-300 mV or so ( 0.1 - 1-3 mA) with nothing else connected to the RF output. If you touch the point by your finger, the output will drop a bit.
3. Connect the open end of your shorted cable to the "hot" point: the detected output can swing high or low. Cut the open cable end by one inch and repeat. After several trials, you will see NO change at all.
This makes your cable exactly a quarter-wavelength long; it has a very high impedance (in theory, infinite) compared to 50 or 75 Ohms of the cable.
4. If you try the same with a half-wavelength shorted cable, the detected voltage should show a perfect short like placing a screwdriver across the coax. Half-wavelength line repeats other end's impedance. If you leave the other end open, half-wave long cable will have again a high impedance (like the shorted quarter-wave long one).