Welcome to T-F
Experience in dealing with situations like these is what matters. Testing really doesn't do much because real world situations are largely different. I rarely encounter two problems that are exactly the same. Every computer has a different configuration causing different problems etc etc.
I learned most of what I know by messing with my own desktop and fixing and trouble shooting computers of friends and family.
You learn a lot by building your own machine, if you haven't yet. It helps out a lot with both hardware and software troubleshooting because you install all the programs yourself (for the most part) so you know what's where and what it does.
Reading through these forums is also a very educational experience. There are a wide range of subjects for just about everything in every operating system.
Good luck with the future job