How do i come to know beforehand which competitions are rated???
The days are fixed. And the entire schedule is given in the website - though it might be slightly tough to find.
Long contest- First friday of every month. Ends on Next-to-next week’s Monday, and it runs for 10 days.
Cook-off - Usually the next Sunday after Long.
Lunchtime- Last Saturday of the month - we can always prove that such a day will exist in a month and hence LTIME can take place monthly 
If other rated contest will have (Rated) suffix.
If Long or Cook-off or Lunchtime contest is unrated then it will have (Unrated) suffix. xD