[Toroi]

On Foot:
https://sampwiki.blast.hk/wiki/GetPlayerVelocity

In vehicle
https://sampwiki.blast.hk/wiki/GetPlayerVehicleID
https://sampwiki.blast.hk/wiki/GetVehicleVelocity

Although I can't tell you for sure what speeds are high speeds, you'll have to test them out. Keep in mind speeds when falling when on foot may be faster than sprinting so you'll have to test a lot of scenarios to make it work well and avoid as many false-positives as possible.