Android malware still represents the most significant threat to mobile systems. While Machine Learning systems are increasingly used to identify these threats, past studies have revealed that attackers can bypass these detection mechanisms by making subtle changes to Android applications, such as adding specific API calls. These modifications are often referred to as No OPerations (NOP), which ideally should not alter the semantics of the program. However, many NOPs can be spotted and eliminated by refining the app analysis process. This paper proposes a visibility metric that assesses the difficulty in spotting NOPs and similar non-operational codes. We tested our metric on a state-of-the-art, opcode-based deep learning system for Android malware detection. We implemented attacks on the feature and problem spaces and calculated their visibility according to our metric. The attained results show an intriguing trade-off between evasion efficacy and detectability: our metric can be valuable to ensure the real effectiveness of an adversarial attack, also serving as a useful aid to develop better defenses.