I'm not sure I'm understanding you when you say "definitively". Do you mean if I can't be 100% sure about my answer?
The answer to your question is: the same way you determine when it's safe to cross the road on foot. When there are no cars in sight I'm 100% sure it's safe. When there are cars in sight, I'm not 100% sure but I still cross because I've developed judgement about the situation and the risks are acceptable.