How to determine if given “complex” time complexity is $O(n^2)$?


If a given time complexity, such as these:

  1. $ (n + \log n) * \sqrt{n+\log n}$
  2. $ n * (200 + \log^2 n)$
  3. $ (7+n^3)\log(n^5)$

is not determinable by just looking at it whether is it in class $ O(n^2)$ or not, how do I decide? If a time complexity is given, and in it there are more types of expressions (exponential, logarithmic, polinomial, … ) how do I decide which one determines the $ O(n^2)$ or $ O(n\log n)$ or … complexity?