The GetInvalidationList operation will return back a list of all the invalidations done for a distribution for the current and previous billing period. If the list is long, you can paginate it using the MaxItems and Marker parameters

CopyC#

GetInvalidationListRequest request = new GetInvalidationListRequest();
request.DistributionId = distributionId;
GetInvalidationListResponse response = cfcClient.GetInvalidationList(request);

public GetInvalidationListResponse GetInvalidationList(
	GetInvalidationListRequest request
)