How to know you are inside a LXD container?

You have access to a virtual machine, you need to find out what virtualization technology it uses, you can install virt-what

apt instal virt-what

Then run virt-what command, it will show what virtualization technology you are using.

root@mysql-1:~# virt-what
lxc
root@mysql-1:~#

Another way to find is using the command. If you use LXD container, you will see the following files/directories.

root@mysql-1:~# ls -la /dev | grep lx
-r--r--r-- 1 root   root          37 Jun 30 21:15 .lxc-boot-id
drwx--x--x 2 nobody nogroup       40 Jun 30 21:15 .lxd-mounts
drwxr-xr-x 2 nobody nogroup       60 Jul 12 17:13 lxd
root@mysql-1:~# 

Also mount command will show many lxcfs mounts.

See LXD