Was it a Star San or other sanitizer solution in the growler, or just water? If not, just do that in the future and you'll save yourself some headache. You're right, you won't know until you taste it (or there are obvious signs in the krausen), but using a sanitized blow-off vessel filled with sanitizer is the safest bet.
Edit: But yes, for now rdwhahb!
Primary: Humulus Congestus
Bottled: Downdraught Imperial Stout, Barrel-Aged Residence Time Barleywine, Sea Smoke Porter
On Deck: Chocolate Peanut Butter Stout, S'more Porter, Barrel-Aged Residence Time Barleywine