I want to know if my vehicle (Vehicle wrapper) collides with the ground. But I can't figure out any method that to check that.
As far as I remember the vehicle wrapper uses rays (along global -Z) to determine wheel collision. The visible wheels are "display only". This means they do not collide with anything.
That is the reason why they can flip through the floor when the vehicle lays on the side.
I'm not familiar with any feedback from the vehicle wrapper telling if a wheel detects the ground.
I suggest you do a similar thing: Sense with a ray along -Z. The distance should match the wheels radius. When it detects something, you can assume it is ground.
1) Make the vehicle dynamic in the physics tab. And make the ground static. In both of them activate the collision bounds under the same tab. Afterwards press Game --> Show Physics Visualization Now when you press P you are gonna see the collision boxes and when the vehicle drops to touch the ground the vertices that touch it are gonna become yellow.
2)Else add a collision sensor to the vehicle and select the material option and there add the material of the ground. You can just order a python script to print "Touch" everytime the sensor is positive.
3)Else add the same sensor as in (2) and also add a property. When the vehicle touches the ground have the property's value to change and make it visible during play time by activating the (!) next to the property and the Game --> "Show Debug Properties".