I had this circuit once in the exam. I wasn't sure if the 30 ohm resistor was short-circuited. It turned out it was. Is there any definition/trick that would help me be sure if something is short-circuited ?
If you can see a direct connection between both its ends then it is short circuited. You should find it when you are looking for resistors in parallel. That one is in parallel with a bit of wire so you can just ignore it.