Return loss bridge, some theory, how I made mine and some demonstration 

Sweeping the frequency spectrum and see what the return loss is at the resonance frequency. Adjust the antenna and see the real time impact.
Calculate return loss to VSWR.
In case you have a spectrum analyser you can use it with a noise generator as wideband source.
This kind of bridge could also be ordered at AliExpress; search for "Return loss bridge". It is on a PCB.