Hi
In a nutshell
The talk pin of a straight through is connected to a talk pin and the recieve pin is connected to a recieve pin, which means that it wouldnt work with 2 computers, however a cross over is wired with the talk pin connected to a receive pin and a receive pin connected to a talk pin.
To tell them apart is easy, look at the glass plug and the coloured wires, in a straight through the colour pattern of the wires is the same at either end, in a crossover some of the wires (i cant remember if its 2 or 4) are connected to different pins so the colour pattern is different.
Its actually a bit more complicated than that as there are 8 wires in a cable but you need a crossover cable to link 2 pc's together.
If you want to use straight through cables then you need a hub, if you only intend to use 2 pc's forever then its a moot point, however should you think that your network may grow over time, then buy a hub as it means that you can easily add more PC's should the time come.
I started with 2 PC's and a crossover cable many years back, I know have 4 laptops on wireless and 3 PC's on cable, I never thought back then that there would ever be more than 2 PC's.
Regards