Although stated elsewhere, you CAN read an output by just inputting the same GPIO pin and get the value returned you just set out before:

GPIO.setup(LED_red, GPIO.OUT) #set Pin LED_red as aoutput

GPIO.output(LED_red, GPIO.HIGH) #set Pin LED_red = HIGH (ON)

GPIO.input(LED_red) returns 1 

As joan's answer has already pointed out the misunderstanding here is likely how to define and use a function in Python. Joan's approach will sure solve the immediate problem and make the program work. I will however address the underlying issue that is not so much related to the Pi but general Python programming.

So how to go about defining and using a function, from here:

• Function blocks begin with the keyword def followed by the function name and parentheses ( ( ) ).
• Any input parameters or arguments should be placed within these parentheses. You can also define parameters inside these parentheses.
• The first statement of a function can be an optional statement - the documentation string of the function or docstring.
• The code block within every function starts with a colon (:) and is indented.
• The statement return [expression] exits a function, optionally passing back an expression to the caller. A return statement with no arguments is the same as return None.

So instead of

ventein = GPIO.output(20, 1)

this is what you're looking for:

def ventein():
    GPIO.output(20, 1)

Note the def keyword, the function name (ventein), the parentheses (empty in this case as no arguments are required), and the colon. The final return is ommitted in this case as it is optional if you do not want to pass anything back to the caller.

And when calling this new function instead of:

try:
    while True:
        if murotemp > 20:
            ventein

it should read:

try:
    while True:
        if murotemp > 20:
            ventein()

import RPi.GPIO as GPIO

GPIO.setup(26, 'output')

# Switch on
GPIO.output(26, GPIO.HIGH)

# To read the state
state = GPIO.input(26)
if state:
   print('on')
else:
   print('off')