I agree that generally your cannot be absolutely sure, if no 73 to your RR73. On the other hand, if you see (decode) all messages from that station to whom you sent RR73 and there is no sending or especially a CQ from that station in the next timeslot, then he is following the 'Rule' that you have logged the QSO and there is no need to resent R... The TU; would be fine in that situation. I got a feeling that it is generated, if you have logged the previous call before your start to send a reply to somebody else. The use of automatic logging may be very relevant in that respect.
I noted that you should not reply to somebody else before logging the previous QSO as that seems to generate the answer using the same serial number as the not yet logged QSO.
Of course situation is less clear, if he is replying to somebody else in the next timeslot (without TU;...) especially when there has been already some repetitions or you don't decode that message. If it is question of an important (rare) multiplier, then you may choose to spend potentially many additional timeslots to get a 73 back. You may try RRR instead as that should tell a not yet logged situation.
73, Reino oh3mA