Assuming WWI or 2-ish technology: just use the mechanisms they already used and had for these types of weapons:
Trip Wires or Electronic fuses
Trip wires were commonly used with mines as an activation mechanism. This concept works with a hoover craft as well. The disadvantage of trip wire was it was easier to see than a standard pressure sensor and as the location of the "trip wire" might need to be altered (raised) this disadvantage would increase. However, its still possible as one could disguise them in the barbed wire which littered the "deadman's land" fields that trench warefare used. I would also set this up as a daisy chain of mines to improve the chances of disabling or even just hitting this quick vehicle.
On a similar note, which would be more accurate but also more dangerous for troops. 477
You could set up a electronic fuse (think the old TNT "plunger") which a solider would trigger once the vehicle was over the mines.
Radio Activated Mines
This became more of an issue in modern warefare (and with IDEs) but radio activated mines are a real thing. The radio technology of WWI required pretty big comm equipment which was usually transported by horse or mule1 but, as trench warefare was largely static, it is conceivable that a few stations could be setup which allow for activating a mine. The range and low development of radio technology would make these much less reliable than fuses but the ability to be farther away and not have to disguise wires themselves certainly makes this an interesting tactical option.
Magnetic Mines
These were actually developed during WWI but did not see much action until WWII (and then only as a Naval weapon)2. However, if these hovercraft are seen as a potent threat and weapon it makes sense that this weapons development would be accelerated and a ground-based version developed.
1: "Golden Age of Radio in the US", Radio on the Frontlines exhibition. Digital Public Library of America.
2: <PERSON>, <PERSON>, "Naval Weapons of World War Two" (London, UK: Conway Maritime Press, 1985) 477