How do Greeks distinguish using Greek letters as a variable, or as a mathematical number such as Pi?
Pretty much every Greek letter is used as a mathematical variable. How would you differentiate between using a Greek letter as a specific variable (Such as μ for apple, as μήλο means apple) or as a general variable like Theta (The known degree of a right triangle) or Pi (3.14159265359)?
It's usual that some letters are used for certain variables. θ,φ for angles, χ,ψ,ζ for unknown variables of equations, the constant π=3,14159 , ρ=radius etc. It's not common that you need all those variables in every problem, though. So, if you don't deal with angles at all, for example, you can use θ or φ for whatever you specify. Θ in physics for example can mean temperature (and not angle, of course). I hope I answered the question well, I am not sure if this is what you meant. ;)