I looked up the same thing once, but not out of any interest to work for dell, just to get a general feel of what's required as 'a techy'. (But I must mention that I never pursued that trail. I'm not a qualified Technician).
All you need to know is correct terminology of components, a good understanding of different ports and adapters, and compatibility across different components, and of course an understanding of certain software, partitioning, RAID, etc.
Basically, if you feel confident that you could fix any hardware issue on a faulty computer, replace components with compatible upgrades/alternatives, and carry out operations such as running ghost images across multiple machines, and setting up / repairing RAID'ed drives.
As for diagnostics, you don't need to be able to diagnose. To carry out a replacement or repair, Dell require that their own diagnostics program declare that there is a fault with a piece of hardware before it is replaced. And I'm not sure about more modern machines but certainly when I was interested in it, there were onboard lights inside the casing which in combinations would give simple diagnostics too. I learnt that little part when doing some voluntary technician work for my county council.