Generally incompatibilities are easier to find than it is to confirm compatibility.
What parts are you trying to find out if they are compatible? For motherboard/CPU check the CPU support list. Most memory as long as its within the voltage and speed range listed on the board will work. Graphics card are pretty compatible as long as you have the right slot.
If you are trying to build a system, start with the easy choices. Hard drive (SATA3), PSU 750 or 800, case (I like the Antec 600 and 900), and video card. The harder choice come with picking the CPU. The reason is the rest of the parts are specific to this choice. When I built this spring it was between the i7-2600k and the current AMD Phenom x6. Which ever way you go the MOBO, Cooler, and RAM are specific to that choice. Once you decide on the CPU, look at the Mobo's. When you have that, go to the Mfg's website and find the RAM's they recommend a pick from that.
...and tada...you have a system ready to buy and build. No compat issues.
You Identify both components then research, like if its a graphics card and motherboard you would first find the mobo or card on the net, lets go with the mobo first, we 'know' its an AsRock m3a770de so we go to newegg or some site that sells them, and read the graphics part of the specs sheet, it staes theres a pciex16 lane for graphics, so we know the card required will have to be a pcie card, not an agp card,
or we go to the graphics card, Sapphire 5770, specs tell us its a pcie graphics card, so you then look at the mobo and check it has a x16 pcie lane
again, what is it you are actually trying to achieve here? we could probably help
I am trying to build a computer that will be able to seamlessly play Skyrim and Battlefield 3, and also I need to be able to swap out components to upgrade, pending further developments in physics engines and such. I'm not really a hardcore gamer, but I do love the immersive experience in these sorts of games, and snow that lags really detracts from the experience. I'm willing to spend up to and including about $700-$750 max on this project, not including building costs (What's a good hourly rate for that anyway?) I'm hellbent on getting quality components (probably from NewEgg) but have no means of knowing whether or not the pieces I want to order will work together.
I also recommend coming up with your own build first, so people know roughly what your looking at, then you'll get plenty of support from people suggesting cheaper components/better deals and overall end up with the best performance vs your budget
Ill also strongly recommend you consider building it yourself. You'll save a nice chunk of money, and its really not quite as hard as it sounds. NewEgg do a great 3-part build series on youtube from start to finish showing exactly how its done. Have a watch of it and see if it boosts your confidence in building your own rig. If you still then really really dont want to, go down the having it built for you route.