How to differentiate a mounted usb stick vs plugged in but not mounted usb stick.

I have a bootable usb live stick plugged in, it is in /dev/sdd

Code:$lsblk
sdd 8:48 1 961M 0 disk
sdd1 8:49 1 7,3M 0 partBut i can't access to it.. i suppose it is not mounted ??
but it is listed in /dev/sdd, ... so it is mounted ??
So is it mounted or not mounted ?
I am confused..

Pls tell me how to know if a usb device is mounted or not ? I need to get this part cleared, because i can't be sure of it.

Thx