I'm about to build my first computer (ordered everything, just waiting for stuff to arrive from newegg) and I keep reading posts from people who say they're able to keep their system/cpu at such and such temperatures...I'm wondering how exactly do you determine the temperatures? Do you use some sort of thermometer? Thanks.
To check your graphics card and processor temperatures you can use bios (accessed during the system startup) or use software like realtemp, coretemp, everest and many more.
If this is going to be your first computer then you should not worry about temperatures since you should only worry about them when you overclock your processor (raising it's frequency above it's official speed). Though you should clean your pc every 3 months or so to keep the cpu and vga cooler clean, otherwise dust will begin to build up and it will drastically reduce their cooling capabilities resulting in high temperatures.
Btw... Did you asked for advice to anyone before ordering your components? I am just asking this because since this is your first build you may need help about components compatibility, especially when it comes to motherboard and cpu's and also when it comes to coolers in case you ordered one, you should also keep in mind about purchasing sata 3 hard drives (not all motherboards support them) and it's very important to always get a nice psu, it's power it's not important at all, it can be more or less powerful depending on your system (though it is absolutely crucial to be aware of what kind of power you are going to need) but there are features you should always look for like active pfc to prevent your system to be almost completely destroyed by peak currents and other energy anomalys.
Feel free to ask whatever you need to know about computers... though when were noobs we don't even know what we need to know sometimes. Just make sure to at least get some advice with us before building your rig, i am sure you could use some, unless, of course, you are not the one who is going to assemble it, still, even if your not the one doing it you should pay attention to who is going to do it to learn something, pc's are in my humble opinion much better than any gaming console but if you don't treat them right they are going to give you hell.
Cryptor, you can click his name and see his posts . You will see he has a system thread posted a week ago. And I would disagree, with any homebuilt you need to check temps during the first boot. Also sata 3 is backwards compatible, it's the 2tb+ that will have compatibly issues with older bios as they can't read that large of a hdd.
I second the use of Hardware Monitor (aka HWMonitor) - is a no-fuss no-muss tool. It will display data for all compatible hardware, which is typically:
* CPU temperatures (possibly voltages)
* HDD temperatures
* Motherboard temperatures (and PWM fan speeds if applicable)
* GPU temperatures
Bear in mind that for some graphics cards, you'll have to install the drivers from AMD/Nvidia, as the ones baked into Windows itself will not expose temperature information to HW Monitor.
The people who make HWMonitor also have a tool called CPU-Z, which gives you visibility of CPU, motherboard and memory information (speeds, etc.)
For the GPU, you can use GPU-Z (from techpowerup), which gives you visibility of GPU information, and dynamic data about GPU temperatures, clock speeds, etc.
As k1114 has said, you need to check temperatures (primarily CPU, then other sensors) on your first boot. CPU temperatures will indicate whether your application of thermal paste was correct or not. The other temperatures will hint at whether airflow is correct and appropriate. More importantly, it will also act as a baseline for you as to what are "normal" temps for your PC. This ties in to what CryptorX said, where periodic cleaning is required (and the need for which will be highlighted if your temps are higher than the baseline).