There's no warning and it will just snap. However if it's about to snap some rust or at least cracked undercoating should be visible but there are no guarantees. The best way is to peel off some of the undercoating to have a better look, but you have to be prepared to patch it back up so water doesn't get in.

__________________
2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL