Using ldd
ldd --version
This should return a result similar to (though your version may vary):
ldd (GNU libc) 2.20
Copyright (C) 2012 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Written by Roland McGrath and Ulrich Drepper.
In this case, version 2.20 is installed.
Using yum
yum list glibc
This should return a result similar to:
Installed Packages
glibc.x86_64 2.20-5.fc21 @anaconda
Available Packages
glibc.i686 2.20-5.fc21 fedora
In this case, 2.20-5.fc21 is the package installed.
