In the rules, it mentions that you cannot guess the Mejai, which is fair.

However, is a lack of evidence enough to consider it confirmed?

For example, both Dekan and Necreph got Destroy Objects, Projects Voice and Eletronic Disturbance, and while both have Radioactivity, only Necreph has Metallic Signature.

If I find an ectoplasm after getting the object, voice and eletronic, and test it for Metallic Signature, if it doesnt react is it enough for me to prove thats Dekan or I do need another evidence?

Not mod but would be pretty sure you have to have some evidence that proves it's X mejai without a doubt (aka having only 1 option in the scroll list). Just because one thing isn't happening right at that moment doesn't mean that it confirms it's X mejai. Vocal response would be the best example in terms of that. Just because it's not saying anything back after a handful of tries could just mean you're unlucky.

The above reply is correct. The absence of evidence cannot be used to determine the mejai as per current rules. This also includes not getting metallic or radioactivity from ectoplasm. Maybe certain exceptions can be added in the future, but for now, that's how it is.

I said about metallic signature which is not a chance when you measure an ectoplasm. If it doesnt have metallic it wont ever have, but I get the point. Thanks!