There are a zillion ways to detect Qubes OS if its tooling is installed inside the VM. A StandaloneVM is not necessarily any different to an AppVM in this regard.
Telegram Desktop uses this library to collect system information. Among other things it looks at how the window manager identifies itself, which in case of an integrated VM’s VM-side window manager is simply “Qubes”. You can do the same thing manually (in the VM) like:
$ sudo apt install x11-utils
$ xprop -root _NET_SUPPORTING_WM_CHECK
_NET_SUPPORTING_WM_CHECK(WINDOW): window id # 0x400002
$ xprop -id 0x400002 _NET_WM_NAME
_NET_WM_NAME(UTF8_STRING) = "Qubes"