For starters, you could run the following two simulations:
apt-get install -s gtk3-engines*
apt-get install -s gtk2-engines*
These commands should indicate engines available to you.
Then you could run:
dpkg --get-selections > ~/Desktop/installed-software
to get a list of software installed on your system and search that file for
engines. I did that and have just the following engines because my OS is Lubuntu 12.10 and I haven't installed any extra themes:
You may also want to look at basic theme engines for GTK+:
GTK-engines is a collection of basic theme engines for GTK+.
The GTK-engines provide basic themes for GTK. Themes control the user
interface, providing icons, shadows, text layout, color schemes,
window decoration and more.
The basic theme engines are the ones that GNOME produce, though users
can install (or write and install) others.
The basic theme engines are very flexible, so many themes can use a
single engine but still get a unique look and feel.
And from reading here and there, I get the impression that one has to be a little careful in ensuring that the themes one tries matches the GNOME version!
As for where exactly the "engines" are located, I honestly don't know but you could see if you have something like