How do you tell when jam is set, what is the wrinkle test? ?

when you seal the jars does the condensation not cause bacteria i had also had a half jar so it has more condensation is this alright sealed straight away. when is it ok to eat.
Update: i was told to put hot jam in hot jars and seal A.S.A.P will the other jars keep ok. so next time heat jars and let them go cold the put hot jam in and seal right a way. jam right up to the top. is this right, any more tips thanks
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