how is skill or talent measured for motorsports like F1 or Rally?

consistency of lap/stage times, number of times you go off the course/track,  number and diffculty of overtaking manouveres, being consistently good in all the above in all types of weather conditions ALL compared as much as possible, with other compettitors in simlarly matched machinery over extended time periods.

I'd have thought that the result of the F1 race yesterday would have told you there has to be a combination of a good reliable car AND a talented driver.   Did you see the condition of the tyres on Hamilton's car at the end of the race - they were virtually slicks in the middle and how he kept the car on the track has to be down to skill.   And what was Bottas doing meanwhile!!

The driver can make the difference. Most racing there are at least 2 cars that are the same. In F1 every team theoretically has the same car. The difference between their finishing position is down to the skill of the driver. In F1 the only real measuring stick of a driver is their teammate. So this season we know Lewis is clearly better than Bottas, Max is better than Albon, Perez ahead of stroll etc.

In Rally you have more drivers in the same car, so you judge against more drivers.

By results. You win, you got skill or talent.