I have a MFJ-854, which will check for common mode. But as a whole most people cannot check for common mode current. They design it out of the system.
Common mode will always be a potential issue, but especially so with small antennas that do not have good earthing systems. As in it's simplest form, common mode is caused when a antenna does not radiate efficiently & throws some of the current back down the outside of the coax.
There are times when stopping the common mode on the coax is desirable, for example in Windom antennas where a properly designed Windom, uses a section of the coax as part of the antenna. But that is using a measured length of coax before the ferrites.
As a whole putting ferrites or a balun at the feed point or before the shack is due to an inefficient antenna.
So the best way to sort common mode is to use a decent antenna & a good counterpoise & earthing system, but that is not always possible or on longer wavelengths achievable.
At 27 MHz you should be able to avoid too many problems. http://www.k0bg.com/common.html