I call this one!

Yes, scales CAN BE used to construct riffs and solos in songs and very often are. Other tools at one's disposal are modes and arpeggios, but since you didn't ask about those, I won't discuss them. I'm not sure which scales you know and which you don't know, so here are a few basic scales in Am/Cmaj:

A natural minor
A-B-C-D-E-F-G-A

C major
C-D-E-F-G-A-B-C

Am Pentatonic:
A-C-D-E-G-A

These scales can be used to construct hott lixx and whatnot. Anyhow, on to identifying key.

The key of a song is merely what notes and scales are available. To identify this, we must know the order of sharps and flats, which are as follows:

Sharps:
F-C-G-D-A-E-B
(Fat Cats Go Down Alleys Eating Babies [or Birds, take your pick])

Flats:
B-E-A-D-G-C-F
(Beatrice Eats All Dead Goat Child Fetuses [ok, that was kind of off the top of my head because it's just order of sharps in reverse])

This means that if a song has one sharp in the key signature, that sharp is F, if it has four sharps, they are F-C-G-D, get it? Oh, and you can't mix sharps and flats or the music theory gods will strike you down where you stand. The amount of sharps or flats in the key signature determines the key, and therefore, the scales and notes that are available to you. For instance, the Eminor scale contains one sharp, this means that the notes in the Em key signature are:

E-F#-G-A-B-C-D-E

Here's a quick rundown of key signatures check it out:

Cmaj/Am: no sharps, no flats

Gmaj/Em: 1 sharp

Dmaj/Bm: 2 sharps

Amaj/F#m: 3 sharps

Emaj/C#m: 4 sharps

Bmaj/G#m: 5 sharps

F#maj/D#m: 6 sharps

C#maj/A#m: 7 sharps

Fmaj/Dm: 1 flat

Bbmaj/Gm: 2 flats

Ebmaj/Cm: 3 flats

Abmaj/Fm: 4 flats

Dbmaj/Bbm: 5flats

Gbmaj/Ebm: 6 flats

Cbmaj/Abm: 7 flats

There you have it, those are the notes available or not available in any given key. From there you can determine what chords work and what scales to use.

That was AMAZING!!! Cigarette?