The Locked Box Test is an informal guideline used to determine an object's most appropriate Object Class. It goes like this:
If you lock it in a box, leave it alone, and nothing bad will happen, then it's probably Safe.If you lock it in a box, leave it alone, and you're not entirely sure what will happen, then it's probably Euclid.If you lock it in a box, leave it alone, and it easily escapes, then it's probably Keter.If it is the box, then it's probably Thaumiel.If you can't fit it in a box and it's about to end the world, then it's probably Apollyon.If you could have locked it in a box but chose not to, then it's probably Archon.
Note that as a special consideration, something that is autonomous, alive, and/or sapient is almost always at least Euclid-class. That is, if you lock a living thing in a box and forget about it, it will eventually suffocate or starve to death, and that's not a good outcome. Something that is intelligent could also end up being smart enough to outwit its containment procedures and/or stop cooperating with the Foundation's attempts to contain it, making it more dangerous than it otherwise might be.
What if I find an SCP that is the wrong Object Class?
The Object Classes are intentionally left vague as to not limit the author's creative freedom; a rigid, defined system of classes might hinder an author's ability to write the way they would want to, and as such multiple proposals to create better-defined systems in the past have been turned down by SCP Wiki staff.
If you find an SCP article that you feel might be inappropriately classified, feel free to raise discussion on the topic and see what other community members think. If the explanation is not to your satisfaction, then feel free to express your opinion on the matter and vote accordingly on the page.
If an SCP is very dangerous should its Object Class be higher?
No, danger does not really affect an SCP's Object Class. As has been reiterated several times above this, an item's Object Class is more based on the difficulty of containment rather than the danger it otherwise poses. For example, a button that can destroy the entire universe when it's pressed would be safe, whereas a cat who randomly switches places with another cat anywhere on earth would be considered Keter.
