Certain lines of high whites and American Blues are prone to megacolon, but there are no visual indicators. The only sure way to find if your line carries megacolon or not is to inbred (daughter x father, mother x son, brother x sister).

If you like high whites but don't want to go through a lot of trouble to find out if your line is 'clean' or not, I'd just breed the high whites to self (solid) colored rats. It will greatly reduce the chance of megacolon and you will still (likely) throw high whites.