when do I check one-sided p-value and when to check two-sided one?

This depends on what your H0 and H1 are. If you have for example a situation in which H0 is that a variable (let's call it X) is 15 or less, while H1 is that X > 15, then you only want to use a one-sided test. However, if H0 is that X = 15, while H1 is that X != 15, you would use a two-sided test because in this case you don't care if it is higher or lower, just that it is different.

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