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.
They are all compatible. Both have the same mount and sensor size.
On most DSLR brands other than Nikon, that is all you need to know.
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.
ALL of the lens that you can currently use on your XT can be used on the Canon 7D.
Reason: they have the same mount. If it doesn't work on the XT it will not work on the 7D
All your lenses used on the XT (350D?) can be used on the 7D. I should know - I went from a 350D to a 7D and took all my lenses with me ;-)