The other methods here didn’t work for me, so here’s what does work in Ubuntu 12.04 ‘precise’.
On Ubuntu and other Debian-derived platforms, dpkg is the typical way to get software package versions. For more recent versions than the one that @Tio refers to, use
dpkg -l | grep libopencv
If you have the development packages installed, like libopencv-core-dev, you’ll probably have .pc files and can use pkg-config:
pkg-config --modversion opencv