I've been doing a lot of research on such theories lately and these are all I've found so far:
Liouvilles criterion (here)
Lindemann-Weierstrass theorem (here)
Gelfond-Schneider theorem (here)
Brownawell-Waldschmidt theorem (here or here)
Schanuels conjecture (no proof yet) (here)
Six and five exponentials theorem (here)
Bakers theorem (here)
Roths theorem (here)
Or theorems such as:
If you take two transcendental numbers, $a$ and $b$, then at least one of $ab$ and $a+b$ is transcendental (here)
or
$x^{x^x}$ is transcencental if $x\in\mathbb{Q}, x\notin\mathbb{N},x>0$ (here)
What other methods show that a number is transcendental besides these ones? If you know any other proven or unproven theorems it would be nice to share them.
Edit 1
K.Dilcher and K.B. Stolarsky (here)
Carl Ludwig Siegel (here)
Edit 2
The Rogers-Ramanujan continued fraction (here) is transcendental for all algebraic $-1<q<1$
All the transcendental functions (here)
Edit 3
All about trancendental Infinite Products (here) (here) and (here)