it'd be like any other infection...it'd need to look wrong (mold forming, or very obvious weirdness like 'stringy' stuff in the starter), or smell wrong.
as long as you sanitize your washing vessels/jars, hands, and used boiled/cooled water for the washing, it'll be as infection free as the original beer.
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10