Contacting previous employers

Hello! I've been reading that when job applications ask to contact employers, it's best to put yes, at least for previous employers. I didn't have any problems with my previous supervisors, I just didn't think at the time to ask if they would be okay being references. In this case, do I need to reach out and ask if they would be okay being contacted, or would it be fine to just check off yes (since it seems to be a pretty standard thing)?

Thanks!

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