You can find all your FB page posts using "Choose Existing Post" option on "Creative" step of campaign creation or selecting a running campaign and going to creatives tab. 

However you may have a hard time to go through all your page posts therefore another option is to find a FB post using their post ID. You can find post IDs by going to Ads Manager and then going the Page Posts part from the main menu. There you can find the IDs of the posts listed.

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