That is just it, you really can't know.
Any PSU, even the best ones, can go BANG and there goes the PSU and there is a decent chance that it takes down something it is connected to as well.
You never know when/if this is going to happen.
Sometimes they go peacefully, they just go to sleep and never wake up again. If this happens parts are not usually damaged by it.
The better ones are just more likely not to have this happen and they are much more likely to last longer in general.
Say you only need like 300w for the system and you get the generic 750w. It may only really put out 350 - 400w and maybe only for a year or two before it kicks the bucket.
The Corsair CX 600w might put out a max of 700w and might last for 5 years.
You are really just playing the percentages here. Do you maximally manage the risk by buying a more expensive brand name PSU or do you accept the risk and hope you don't pay for it later?
It is up to each individual's tolerance level.
That being said, I can't in good conscience recommend to anyone that they choose a generic brand PSU.
That being said, a glaring warning sign that the PSU is on the way down is a computer not being able to wake up from in sleep mode. Waking up from sleep mode means drawing an increased amount of power from the line listed +5vsb. This line keeps a tiny bit of power going when the computer is sleeping so that it can remember the previous state.
Waking up requires increasing the amount of power above that minimal level.
Often times the +5vsb line is the first thing to go on a PSU, so if you can't wake up from sleep mode that usually means you should replace the PSU right away, especially if it has been working for more than a year without problems and this just starts suddenly.
Really, power problems can manifest in many different hard to diagnose ways and they can very easily look like problems with any other part. That is partly why I always suggest people get PSUs I can trust, because if they get a problem later its really hard to rule out that it is a power problem.
Anyway, you might just want to plan on replacing it in a year or two and start looking for good deals on PSUs around the 1 year mark. If you want to move forward with the generic 750w, that is definitely something I would advise.
You never know if you will have any warning signs, it could just up and die out of nowhere. It happens more than you might think.
Anyway, GL on the computer. I hope it works out well.