I am looking to purchase a 7d and currently have an XT. How do I find out if the lenses I have are compatible with the my new potential camera?
Canon EOS bodies can accept two types of lenses, EF and EF-S mount. EF work on all and EF-S only work on crop sensor bodies. Both the XT and 7D are crop sensors so they accept either EF or EFS lenses. If you were to purchase a full frame body such as a 5D mrk II or 1d X then you could only use EF lenses on those bodies.
All Canon lenses (EF & EF-S mount) should be compatible. In fact, some of the correction features like peripheral illumination correction will be enabled for lenses that came out between the XT & 7D.
However, 3rd party lenses like those made by Sigma & Tamron may not work.
Quoting from a review of the Sigma 17-50mm lens:
My standard disclaimer: There are some potential issues with third party lenses. Since Sigma reverse engineers (vs. licenses) manufacturer AF routines, there is always the possibility that a DSLR body might not support a (likely older) third party lens. Sometimes a lens can be made compatible by the manufacturer, sometimes not.