QUICK QUESTION!

You need to narrow down the problem you are experiencing. Your open-ended question has an infinite number of possibilities. Please clarify. Also look at the many guides found at the link below to help:

http://www.tomshardware.com/forum/288241-31-read-postin...

Oh, I haven't ordered my parts yet.

It's just that, while I was reading some feedbacks on some parts, It said it was defective, and I was wondering how would I be able to know which part was defective.

Without having the device and installing it properly, you can't determine if defective or not. If, when researching a device, you see lots of complaints about many similar defects, look for another device/component.

If you want input from the forum on the devices you are thinking about buying, just list them and I am sure you will get plenty of good advice.

But what if I installed everything, and it doesn't turn on?

How will I know if the PSU, CPU, or the motherboard is defective?

And in this kind of scenario, where multiple items are suspicious, what do I do?

Firstly, you calm down and stop worrying yourself so much  
then you read section four of this http://www.tomshardware.co.uk/forum/288241-13-read-post...
and if you do encounter any issues with your build, read section three and post back on here, we can help identify any problems and suggest solutions accordingly,
Moto

Between the 3(cpu, mobo and psu) it's very hard to determine which is faulty. You need to replace each part systematically to see which is broken. PSUs are easy to come by so that problem should easily be fixed, and then between the mobo and cpu I was in that situation once so I contacted the manufacturer once and they claimed it was the mobo and they were right so they replaced it for free.

But as everyone here said no need to worry, parts are made to work  .

Alright, Thank you guys  

Best answer selected by jineus.