How do you know if the lenses you have will fit a camera you want to buy.

I have a 35mm Canon Rebel Ti and lenses 35-80mm and a 80-200mm

I am interested in investing in a Canon Rebel XTi digital camera, but want to keep my lenses... how can you tell if the lenses I have will fit the Rebel XTi? I don't really have a camera shop I can just drive to to check them out and see if they fit. These are canon lenses but was just wondering... how do you know, do most canon lenses fit all Canon Cameras? Is there a number somewhere on the lens to know if it is compatible to the camera?

Can anyone help me?

Your lenses will fit the Rebel XTI (400D). BUT they will work diffrently.
The size of a single frame of 35mm film is roughly 36mm x 24mm. Now the imaging sensor on (CMOS) on the XTi is roughly 22 mm x 15 mm. About 1.6 times smaller in dimensions. So the image that will beformed on the sensor using your existing lenses will be cropped by 1.6 times at the margins effectively giving you a magnified print by 1.6times. In other words you can say that your 35- 80 mm lens will work with an effective focal length of 56-128 mm (35 x 1.6 & 80x1.6). So that is no good because you will always have magnified image by 1.6.
So the lenses that Canon makes for these cameras are labelled AF-S lenses instead of AF. The routine basic lens that can be used with the XTi is 18-55mm AF-S lens giving you an effective focal length of 28.8-88mm (18x1.6 & 55x1.6). The AF-S lenses will not fit the your original 35mm camera but it is possible to fit the lens of 35mm on XTi.
If you still want to use your old lenses with the same utility the you should go for the Canon EOS 5D and upwards. The imaging sensor of these cameras is the size of your 35mm film.