We present a numerical investigation of effective chipless tags for radio-frequency-identification (RFID) applications. Chipless tags have been introduced recently as alternatives to standard tags with microchips. While they can significantly reduce the overall cost of RFID systems by eliminating microchips and procedures to mount them on tags, chipless tags bring new challenges, especially in terms of identification reliability. We focus on tag structures that consist of resonators and consider alternative scenarios to find out potential misidentification cases. We also present the robustness of resonator-type elements in terms of fabrication errors, as well as array strategies to significantly increase electromagnetic responses of tags at the cost of reduced compactness.