Isolate a single fiber, bend it slightly beyond its bend radius, measure the light that escapes. This will appear as a slight reduction in signal strength, but won't be conclusively identifiable as a tap. https://www.thefoa.org/tech/ref/appln/tap-fiber.html
For the Celtic - Norse cable now is probably the ideal time to tap it as it does not appear to be operational yet.
For other cables brining them down with some strategic use of an anchor and tapping before it is repaired would probably be very tricky to detect.