I have been bequeathed a couple of external antennas ... the stick/wand type that screws into a modem or wifi access point etc.

 

One is 195mm in length, the other 375mm.  I was told that the short one was 10dBi, and the other possibly double that.

 

There are no markings on the units, so is there a way to test and/or ascertain just what dBi gain they may be?