Internet congestion control remains a fundamental challenge, and recent learning-based congestion control algorithms (CCAs) have shown potential in optimizing network performance. However, their reliance on heuristically chosen input signals often leads to suboptimal behavior across diverse network conditions. In this paper, we identify the root cause as the lack of signal distinguishability-the ability of signals to reflect meaningful differences in network states. To address this, we propose Learn-to-Probe (LTP), a novel signal engineering paradigm that actively generates distinguishable network signals to improve the learning process. LTP (i) employs Bayesian filtering to accurately estimate network states from historical signals, and (ii) introduces an intrinsic reinforcement learning reward that encourages the flow to probe the network, inducing signal sequences that minimize the estimation uncertainty in (i). This probing behavior naturally enhances signal distinguishability, enabling the learning model to make more informed decisions. Extensive evaluations show that LTP consistently achieves high link utilization, low queuing delay, and stable convergence across diverse environments. Our results underscore the importance of signal distinguishability and offer a new direction for robust, adaptive congestion control.