If you used the e-file system, you will get confirmation of the filing. If you mailed it, you need to use one of the formats offered by the USPS which enables you to get a receipt and signature of delivery. Priority mail is one such option. You should never simply drop them in the mail for ordinary delivery because you won't have a receipt of having filed. If you put a check in your envelope and it was cashed, then obviously they got your return but do not use ordinary mail in the future. The same would apply if you got a refund either by check or direct deposit. They would not be issuing you any refund if they had not gotten your returns.
