Most slip and fall cases come down to notice: whether the property owner knew about the hazard, or should have discovered it through reasonable care. You do not always have to prove someone actually saw the spill — it can be enough that it existed long enough that a careful owner would have found and fixed it.
That second kind — constructive notice — is often proven with surveillance video showing how long the hazard was present, cleaning and inspection logs showing when the area was last checked, and witness accounts. Actual notice can come from prior complaints, employee statements, or records showing the owner already knew.
Because this evidence — especially video — can be overwritten or lost within days, acting quickly to preserve it is one of the most important things in a slip and fall case. That is a big part of what a lawyer does early on.