Time slips represent an interesting and compelling idea, but in the absence of convincing physical evidence, I don't see any way that they can ever move out of the realm of visionary experiences.
To be clear, I'm not saying that they are necessarily hogwash. Those alleged witnesses who aren't lying, deluded, or simply mistaken may well have experienced something genuinely unusual, and subjective experiences can be deeply important and impactful psychologically. However, ultimately they reveal more about the human mind than about physical reality.
What sort of physical evidence would be convincing? It's hard to think of useful examples that don't require the witness to be aware of the situation and act accordingly. Perhaps burying a modern artifact near a major landmark, making unmistakably modern inscriptions in a hidden place on an easily identifiable structure, or stealing a historical artifact of some importance?
Even if someone has the opportunity to act on such intentions, much can go wrong. Buried objects can be uncovered too early, by random persons, and their presence in the ground from earlier times can be hard to prove without precise stratigraphy.
What kind of mark is unmistakably modern? "Kilroy Was Here" could be mistaken for simple doodles of a face. Perhaps a clear reference to the internet? In any case, marks could be removed over time, and their true age isn't always easy to estimate, so they could have been made more recently.
If time slips are physically real, then artifacts stolen in the past should not exist in the intervening period. But how would historians verify absence? Let's say I produce a missing Fabergé egg to verify a time slip. How would I prove that I didn't just buy it off some family that had been hiding it for the last 100 years?