I need it to determine if hitting ctrl+d would disconnect me from server or just close current screen.

Is it somehow possible to check if I'm right now in screen session?

You can look at the $TERM variable.

echo $TERM

If it's a screen session, the term variable should return "screen".

root@deore:/volumes# echo $TERM
screen

Ctrl-a -d (to exit screen)

root@deore:/volumes# echo $TERM
xterm

Also check: https://stackoverflow.com/questions/3472287/how-do-you-tell-if-the-current-terminal-session-is-in-gnu-screen

You can look at the $STY variable (a variable set by the screen command). If it is not "" then you are in a screen session.

I am in screen

$ echo $STY 
29624.pts-1.iain-10-04
$

I am not in screen

$ echo $STY

$

I have found another solution: Modify your .screenrc, so my screen session looks completely different from normal terminal.

Unless you have changed the default key bindings, you can do Ctrl+a -> Ctrl+t, which will show the time, if you are in screen. This will work even if you have ssh:d away somewhere else, unlike the other suggestions.

The caption command in the ~/.screenrc is a nice way to differentiate a screen session.

I'm personally using this:

$ cat ~/.screenrc
caption always "%{= kc}Screen session on %H (system load: %l)%-28=%{= .m}%D %d.%m.%Y %0c"

It adds a line like this one at the bottom of the screen:

Screen session on gbook (system load: 1,75 1,74 1,68)                   Lun 05.01.2015 13:01

With the first part (system name + load) in green and the date in pink. Useful and hard to miss!

Do a screen -ls. It's going to explicitly indicate Attached versus Detached status.

Example attached:

$ screen -ls | grep tached
3132.pts-0.esavo00      (Attached)

Example detached:

$ screen -ls |grep tached
3132.pts-0.esavo00  (Detached)

screen -ls

to view your sessions and

screen -r sessioninfo

to reconnect to a disconnected one, if detached.

screen -D -r sessioninfo

to reconnect to a disconnected one.

If you are looking at a command line prompt, you can just type something, anything, and hit Ctrl+A. If your cursor jumps to the beginning of the prompt, you're not inside a screen. If you additionally have to hit A, then you are.

Better answer (in my opinion), inspired by this, just type the following:

pstree -s $$

If you get something like this:

systemd───sshd───sshd───bash───screen───screen───bash───pstree

… then you are in screen.

This is true not only for screen, but also for any kind of process (like script, nested bash or other shells) opening a nested shell, and this can even also show nested screen calls (if several not consecutive occurrences exists).