Hardware Trojans are malicious alterations to a circuit. These modifications can be inserted either during the design phase or during the fabrication process. Due to the diversity of Hardware Trojans (HTs), detecting and/or locating them are challenging tasks. Numerous approaches have been proposed to address this problem. Methods based on logic testing consist in trying to activate potential Hardware Trojans in order to detect erroneous outputs during simulation. However, traditional ATPG testing may not be sufficient to detect Hardware Trojans. Hardware Trojans are indeed stealthy in nature i.e. mostly inactive unless they are triggered by a rare value. The activation of a Hardware Trojan is therefore a major concern. In this paper, we propose a procedure to identify circuit sites where a possible HT may be easily inserted. The selection of the sites is based on the assumption that the HT is triggered (i) by signals that have potential rare values, (ii) in paths that are not critical, and (iii) combining multiple gates that are close one to the other in the circuit's layout, and close to available space. This identification is then used to automatically generate test patterns able to excite these sites.