The walkaround technique for a specific aircraft will be stipulated and the person doing it will be expected to perform it thoroughly.
I'd imagine that as the aircraft changes from a PA28 to an A380 the walk-around becomes more of a gross error check than a detailed inspection - the technology/error checking onboard allows that too. You won't physically be able to see the wing upper surfaces, probes, tail, etc. so, all these things will be looked at to a degree, but the focus will probably be more on what can be seen from the ground - panels, leaks, wheel assemblies, etc.
It's traditional and still important - I remember one incident where the probes on an airliner were left taped-up and it led (I think) to a fatal crash. I'm pretty sure it was found out that a thorough walk-around should have picked up the problem.
V1R
