Estimating the complexity of an algorithm by looking at code
This question is rather informal, and you may enjoy reading this.
What you are asking about is a topic discussed often in Computer Science and Mathematics (perhaps all subjects, but my perspective is not so broad) - "developing an intuition". Often, students are looking for some sort of cheat code to solving problems quickly without going through all of the steps. The honest answer is that you can only develop such intuition and speed by doing precisely the opposite - practicing slowly and without any sort of shortcut. 

Practice calculating time complexity - it really isn't too difficult. A quick google search gave me this page, which is pretty informal and may suit the needs of some practice problems. Once you have gone through the process enough times, you will ascend to a sought-after plane of intuition.

Having very little professional knowledge is not an issue. If the example page I sent is too formal, there are plenty of other pages on the web explaining time complexity. You just have to be willing to sit down and put some hours into it (which no different than what a "professional" does to get the same knowledge, except perhaps they pay more for it?).