How do I know my KDE version?

In modern Kubuntu there is an Info Center app which you can find in K menu -> Applications -> System -> Info Center or start from console using kinfocenter. It shows Plasma version, Frameworks version, Qt version and other useful info.

A command-line solution for KDE/Plasma 5:

$ plasmashell --version
plasmashell 5.9.4
$ kf5-config --version
Qt: 5.7.1
KDE Frameworks: 5.31.0
kf5-config: 1.0

Previous answer for older KDE versions:

Start any "standard" KDE program, such as Konsole or Kate and select Help - About KDE

If you want to do the same from the command line, you can invoke any "standard" KDE program with --version (or -v) parameter. The program then will print the version and exit:

# kate --version
Qt: 4.8.6
KDE Development Platform: 4.14.2
Kate: 3.14.2
# konsole --version
Qt: 4.8.6
KDE Development Platform: 4.14.2
Konsole: 2.14.2
# plasma-desktop --version
Qt: 4.8.6
KDE Development Platform: 4.13.3
Plasma Desktop Shell: 4.11.11

On kde5 I have resorted to this in scripts:

dpkg -l | grep plasma-desktop\ | awk '{print $3}' | cut -d':' -f2 | cut -d'-' -f1

Or, shorten version of command:

dpkg -l | awk -F"[:-]" '/plasma-desktop\s/{print $3}'

Open any KDE related program, like Dolphin, Kmail or even System Monitor, not a program like Chrome or Firefox. Then click on the Help option in the menu and then click on About KDE. That will tell your your version.