If everything were to be pulled towards a point at the "top" of the hill, including cars, pendulums, the fluid in spirit levels, and abandoned marbles, then there is a very real sense in which it would be the bottom of the hill!
Gravity can even bend light, and if all the possible ways of measuring something may be influenced by what you are measuring, you need to make decisions about how to interpret your results.. Looked at in sufficient detail, what is a straight line or a slope is determined by mathematicians and the model they choose to use in any given situation.
However, it is easy to establish whether an actual "electric hill" has some special power that attracts things to the "top": check it from every side. Here are two points either side of a hill:
a/\b
If a appears to roll uphill, but b does not, then either the "uphill" aspect is an illusion (likely) or the attractive force only radiates in one direction which would make it a new force, quite unlike magnetism or gravity.
Here is another simple test, with three points on one side of the hill:
/\a b c
If a force such as gravity is pulling a towards the top of the hill, then the effect should be measurable at predictably decreased levels at b and at c.
Change of tack: my wife studied geophysics at university, and one of the experiments they did was to use a gravitometer to measure tiny variations in the strength and direction of gravity in a given area.
By plotting their results, and eliminating "noise" (the effects of slight changes of altitude, the position of the moon, etc.) they were able to find a signal that proved the existence and defined the shape of a (previously known) subterranean area of heavy clay or rock. Gravity does not always pull directly to the centre of the planet. It was however a tiny variation, and would not explain cars appearing to roll up hill!