apt-cache show <packagename>
should include the full version numbers.
Update as this answer has recently seen upvotes:
With more recent releases you can use
apt list <packagename> to get a display of the most pertinent information (version, installation status) in one line or
apt show <packagename> for fuller information as provided by
apt-cache. The other commands (and
aptitude too if installed) still work as before.
Be sure to heed the warning emitted to stderr¹ if you pipe the output of apt to other commands, if you use it in a script or documentation that is intended to be around for a while.
 “apt does not have a stable CLI interface yet. Use with caution in scripts.”
If you just want the version info without the description and such, use
apt-cache policy instead of