If I'm playing a solo how do I know what key it is in and what scale is required for me to play harmonizing thirds?

I understand that the key is the tonic note of the scale but what I start in the middle of a scale that I can't indentify then I wouldn't be able to discover the tonic.

You'd have to look at the other notes in the scale, when youget about 5 or 6 ntoes theres very few scales that it could fit in. Also look at the backing chords for a hint. If you want to harmonise your solo in diatonic thirds just play a note either a m3 or a 3 above the note your harmonizing with, which ever fits into the scale.
Quote by cakemonster91

*chuckle* A peanut. With a face.

Go to your staff paper and re-write this song a half step down so on the paper it'll be like you have a "C" just move it down to a "B#"

Know your theory, then play like you don't.

Write down the notes you are playing and the intervals, and compare them to the Major scale's.
Proud Owner of:

Jackson RR3
Jackson WRMG

Quote by madbasslover
What's the big deal with Gibsons, anyway?
I've heard loads of Gibsons being played before
and they don't sound any more special than
any other guitar.

^UG's King Of Fail.
You know the key by the key signature, and if you want to play a scale that harmonizes thirds you'd use the scale that's a major/minor third above the original scale. So say you wanted to harmonize a major third with G major, you'd play B major. Or if you wanted to harmonize minor third with E minor, you'd do G major.

To figure out what scale you're playing, you need to know the notes of your major scales, your relative minors, and your key signatures.
alright so I said I KNOW that to harmonize you would go up a 3rd.

What I am asking is lets say I WROTE a solo and I wanted to know, using theory, the key of the song...for those of you who said listen...
Being that I am trying to find the key there wouldn't be a key signature...duh.

My question is how do I find the tonic note in a scale if I started on something other than the tonic.
Well I apologize, the wording of your post confused me.

Just look at the notes you're playing. If you see you're playing, E D F# B G A C#, you can tell it is D Major/B Minor because it has 2 sharps.