For example:

if ("PC4 is connected to 5 volts") {
    //Code
}

How may this be accomplished?

The state of the pins is reflected in the PINx registers assuming the pin is configured for input (DDRx bit is zero). I'm pretty sure the Nerdkit guide goes over this, so I would review that. There is also an example in the library.

an example from a project of mine where i assign a pins state to a variable. at which point you can just check to see whether or not the variable is reading 1 or 0. when the pin is receiving vcc(5v) it will read 1 when it is not it will read 0.

button_up = (PINC & (1<<PC5)) >> PC5
if (button_up == 1){}

alternatively you could simply do something like this if you don't need to use the button state more than once per program loop.

if (((PINC & (1<<PC5)) >> PC5) == 1){}

Thanks delinthe that should be very useful for me. Also thanks to bretm for helping too.

Or just

if (PINC & (1 << PC5))

I would be prefer the solution of bretm. As I remember this is exaclty the solution described in the nerdkits guide.