Calculating Round trip time (RTT)?

Question

Computer networks

Hello everybody. Is it possible to calculate RTT without having ack packets in the protocol (Protocol over UDP) ? For example, based on the time between receiving packets from the second party ? Or somehow (without RTT) to know that the other party's network is congested and needs to lower the send rate of the packets
Unfortunately, adding acks (to calculate RTT) is too much of a hassle for this protocol and is considered a remedy if nothing else did not help :)