Plumb lines and spirit levels can not be relied on if you think there is a gravitational anomaly. If the slope runs parallel to a seaview it would be possible to compare a plumb line with the horizon. Otherwise the only reliable way of determining the true horizontal is by careful surveying. If a good topographical map of the area is available it may be sufficient to show which way the land is really sloping. The results will confirm the illusion.
Gravitational anomalies are always very small.
In any case, if there was a gravitational anomaly you should wonder how you would notice it. There would be an equal effect on your sense of balance as there is on any object. The anomaly would not be apparent unless there was a clear view of the sea behind the slope, which there never is. A search on the web turned up a surprising number of examples of this illusion. Most are natural while others have been constructed in theme parks.