[singleshot_]

How would one know perfidy occurred?

The search term that might help here is “previous judgment, pronounced by a regularly constituted court.”

Also: if one is outside of the protection of IHL/LOAC, might other laws protect him?

[bandrami]

This is my fault; I hadn't read the whole thread. There's two acts that constitute perfidy: one is wearing false uniforms or displaying false colors; I agree that isn't usually met with summary execution. The other one is taking back up arms after signalling a surrender. That is absolutely met with summary execution.