In wireless sensor networks, many protocols assume that if node A is able to interfere with node B's packet reception, node B is within node A's communication range. It is also assumed that if node B is within node A's communication range, node A is able to interfere with node B's packet reception from any transmitter. While these assumptions may be useful in protocol design, they are not valid, according to the real experiments we conducted in MICA2 platform. For a strong link that has a high packet delivery ratio, the interference range is observed smaller than the communication range, while for a weak link that has a low packet delivery ratio, the interference range is larger than the communication range. So using communication range information alone is not enough to design real collision-free media access control protocols. This paper presents a radio interference detection protocol (RID) and its variation (RID-B) to detect run-time radio interference relations among nodes. The interference detection results are used to design real collision-free TDMA protocols. With extensive simulations in GlomoSim, and with sensor network application scenarios, we observe that the TDMA which uses the interference detection results has 100% packet delivery ratio, while the traditional TDMA has packet loss up to 60%, in heavy load. In addition to the scheduling-based TDMA protocols, we also explore the application of interference detection on contention-based MAC protocols.