In Acrobat you can read out the Reader Extension information via JavaScript as follows:
this.appRightsValidate({}, 2, false)
It returns an SigInfo object you can get many details from such as:
this.appRightsValidate({}, 2, false).appRightsDocument; // FullSave
this.appRightsValidate({}, 2, false).appRightsForm; // FillIn,SubmitStandalone,Add,Delete
this.appRightsValidate({}, 2, false).appRightsSignature; // Modify
To check if there are any Reader Extensions applied to a PDF:
this.appRightsValidate({}, 1, false); // true or false