How can I tell if my bikes steering has been modified?

In addition to what CCjon says, if you're not a member of the USCA, you're missing out on the Sidecarist magazine. That in itself is worth the price of membership.

if you have a raked front end,usually 5 or 6 degrees, the down tubes will be closer at the top of the neck than at the bottom of the neck. if it is stock the neck and the tubes will be at the same angle. if it is raked, the tubes are kicked out at more of an angle than the neck