A short answer to a complex question - not really.
In western music you usually - but not at all always - end up on the same
chord - where you started playing your tune.
(This rule is so full of exceptions it has to be taken with a big grain of salt)To be certain of what key you're playing in you'd really have to know the names of the notes you're playing.
You'd have to have a at least a bit of knowledge on
how chords are built from notes in the scales you're using.
The problem is that music are all about relationships between different musical building blocks.
The same notes can be used for a variety harmonic scales
depending on how you play them
Example: the pentatonic C-Major scale is excactly the same notes as
the pentatonic A-Minor scale (the basic blues-box on a guitar)
However they sound very different while your playing them in context.
In essence your putting on two different pair of glasses, different musical
color/mood if you will.
Chords in western music are built from notes in the correpsponding scales.
You start by stacking intervals of thirds on top of each other.
( then expand and add other intevals as well for coloring )
Example: A C-Major chord needs to be built from at least three notes from
the C-Major scale - using two third intervals.
The three notes are C - E - G
The interval between C - E -- is a major third
The interval between E - G -- is a minor third
Imagine the intervals as plates.
The C-major chord is a bigplate first with a little plate on top.
The C-minor chord:The C-minor chord is a little plate with a big plate on top.
The three notes is: C - Eb - G
The interval between C - Eb -- is a minor third
The interval between Eb - G -- is a major third
The basic rule is pretty simple, but later it gets complicated but fun.
There's no substitute for knowledge and know how.
Buy some books on the subject and start eating
'Bout LE7 (last question) I dont know
Cheers: Dixiechicken