Depends, if you just need to detect when someone is sitting on a VehicleSeat you could something like this:
local VehicleSeat = script.Parent
VehicleSeat:GetPropertyChangedSignal("Occupant"):Connect(function()
if VehicleSeat.Occupant then
-- if player is sitting
else
-- if player is not sitting
end
end)
but if you need to check if someone is in a certain radius of the vehicle you can use something like ZonePlus (which uses the somewhat new Spatial Query API).