You would have a reaction within the hour after eating peanuts or peanut butter. But short of isolating the suspected ingredient, you can't know with 100% certainty without talking to a doctor and getting tested. If you are that concerned that you may allergic to peanuts go to the doctor.
The most valid way to tell if you have a peanut allergy is history of reaction to peanut protein, confirmed with positive skin prick and blood allergy tests.
Food reactions can happen within minutes or can be delayed for up to several hours. Not all reactions are immediate.
Following is some basic food allergy info from my website:
Hope that helps!
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