I've seen some of slimsurf's work and it is rather good.
To answer your question, its tricky with no example pics (you've got my email address so feel free to send some across) however if the ball is in focus and the player isn't then that isn't back-focussing. Quite the opposite in fact.
I assume you're using 1d bodies? Are you using a different focus point for when the camera is in portrait orientation? This is something i've been experimenting with during the rugby i've been covering.....takes some getting used to, but with custom function tweaking and focus point setting then you're away.
In terms of checking focusing, i checked mine a while back by simply using one shot focussing and focussing on something at the closest focussing distance. On a 300 2.8 that gives about 3cm depth of field to play with and quickly shows whether the focussing is accurate.
Not a scientific way, but worked for me as peace of mind!
Hope this helps a little,
Paul