In open multi-agent systems (MAS), norms are being used to regulate the behavior of the autonomous, heterogeneous and independently designed agents. One of the main challenges on developing normative systems is that norms may be in conflict with each other. Norms are in conflict when the fulfillment of one norm violates the other and vice-versa. In previous works, the conflict checkers consider that conflicts can be detected by simply analyzing pairs of norms. However, there may be conflicts that can only be detected when we analyze several norms together. This work presents a conflict checker capable to detect conflicts between two or more norms at the same time. A new, more expressive normative language, represented by a BNF grammar, was developed to define norms and Conflict Checker was implemented in tool format. Two validation principles were applied: software testing and formal verification. The strategy thus developed emerges as a new syntax for definition and verification of conflicts in MAS.