As was for mentioned, look for weight, if the product does not say how much it is look for the shipping weight that will be pretty close. Anything about 4-6 lbs is a good weight. The heavier the components of the power supply the better quality they are.
You don't have to spend a lot of money for a good power supply the most you would need, as in the cheapest I would go is around 50 to 55$, anything below that usually aren't made with "quality" parts or are very generic power supplies.
To find the right amount of wattage you will need for a computer you can go to a website such as this one and calculate your wattage needs. http://www.jscustompcs.com/power_supply/
However you can use this simple chart to easily figure out what you need.
High End System=520 Watts to 800 Watts Depending
Medium End System=400 watts to 500 Watts Depending
Low End System= 300 Watts to 400 Watts
High End systems
~This includes Servers, Video work stations and Top of the line gamming computers. Or computers with multiple HD and Burners for different needs.
Medium End systems
~Systems much like high end but have mid quality parts and or single uses.
Low end systems
~Only used for light light gaming, word processing, email and internet use.
With medium to higher end systems you will want to have PFC on your power supply. PFC stands for power factor correction. The power coming into the power supply is not always "clean" there can be variances with frequency and spikes that lower the life span of your power supply and can cause damage to parts if it is not corrected. Active PFC is more efficient then Passive but also is more expensive.
Other aspects of Power supplies, are hold up time and efficiency. Hold up time refers to how long a power supply will continue to produce voltages after the AC input has ceased, it is measured in milliseconds. The longer the better.
Efficiency in a power supply is determined how much of the energy it takes in is actually used and is not wasted in other forms, as with hold up time the higher the efficiency the better, 65 to 70 and higher is recommended, The more efficient power supplies save more money on the electric bill. The efficiency for most power supplies is when it is at highest workload or when the most energy is being drawn from it.
~Voltage rails should never straw more then 5% in either direction, that is also what is recommended. If a rail does drop or increase more then 5% of what it should be it can potentially damage and destroy components. This isn't something most companies tell you before you buy it, its something that is bested tested with a Digital Multi Meter (DMM) while the computer is active.
~Obviously Higher systems should have more amps on their rails, for low end systems, with power supplies with only 1 12V rail there should be from 15-17 amps on it, Medium end, 19-24, and 26-35 amps for high end systems. For computers that have a PSU with dual 12V rails or more, 12V1 @ 18-22 or more Amps and for 12V2 the same. The 12V rail is the main concern as it is drawn from the most and is directly connected to motherboard.
As for reliable Brand names
(High Priced: Usually Greater Than 150$ USD)
~PC power and cooling
~Fortron (FSP Fortron, 700 Watt Version)
(Medium priced: Usually Between 70$ And 150$ USD)
(Low Priced: Usually Less Than 70$ USD)
~Forton (FPS Fortron, 400 and 450 Watt Version)
~Rosewill ( I Stress The LOW End Part Here)
Some brands I mentioned have higher end power supplies and some lower end ones but for the most part it holds true.
The brands I stay away from and would never recommend. Ever.
~Thermaltake (The Higher Models Only, See Low Priced Power Supplies)
~Ultra (X-Connects Especially)
Generic ones that come with cases are usually listed at their peak wattage not their continuous wattage, which means the power supply may only be a 320 watt continuous but they list it as a 380 watt Power supply, which is its peak wattage. This way they make it look better then it really is. If you are an overclocker, than you will want to buy from the medium to high priced region. Overclocking puts more strain on the voltage rails. Therefore only high quality Power supplies are recommended. A digital multi meter will assist you during Load Tests. You can check the voltage and amperage readings of your power supply to check for stability.
Those power supplies that look sweet, that glow and flash and are brightly colored are that way for a reason, to sell, they make look awesome but as you will learn these power supplies are only this way cause just about everything else is bad, as in quality of parts, life span and efficiency. There are few power supplies that actually are good performers and look good. They are usually more expensive then people want which is why the cheap crappy ones that look so cool sell a lot. Stay away from these power supplies unless they are made by the name brands I mentioned under good brand names.
~When buying a psu, dont cheap out. Brand names = good quality even though there more expensive. PSUs work like this. (my attempt is probably vague but at least Im trying). If you buy a 500 watt generic PSU and compare it with a ,lets say, 350 watt brand name, the brand name would perform better and last longer. DONT CHEAP OUT WHILE BUYING YOURSELF A PSU!!!!