Bikepedia specs for the 2002 R400 (triple) says 1" threadless, but the picture clearly shows a quill stem with a (presumably) threaded steerer, sho who know what TF is going on there.
https://www.bikepedia.com/Quickbike/...spx?item=37097

In my experience, though, 2002-2003 was pretty late to still be using 1" steerer tubes. Threadless 1" setups were pretty short lived (late 90s - 2001 or so) and Cannondale always seemed to be at the front of trends, so having 1-1/8" would be what I would expect. Most good quality road bikes had gone to threadless integrated 1-1/8" headsets by 2003. It is also possible that Cannondale had leftover 2002 frames thatthey built up and sold as 2003 bikes, or the production run of 2003 frames started in 2002.

Anyhoo, if the OD measures 28.6 mm then it is 1-1/8. If you tried to fit a 1" stem on it you would immediately realize you were using the wrong size. There will be no ambiguity when you try to mount a stem.