The best way to learn whether to up strum or down strum is to repeatedly practice up-strumming and down-strumming on the same chords.
You'll learn over time that there's a slight difference in the sound of each strum, and then you'll be able to recognise those differences in your favourite songs.
If you're attempting to learn how to play your favourite songs with the use of guitar tabs, it may be difficult to distinguish between up and down strums.
Most guitar tabs don't specifically indicate whether you should up or down strum, and again, you'll be able to hear the differences with practice.
Does it matter?
If you're simply starting to learn how to play guitar, you shouldn't worry too much about whether you should be up or down strumming. Instead, focus on being able to play the chords correctly.
Up and down strumming is an important thing to learn over time, but won't make too much difference to the sound you produce as a beginner.
Here's a pretty awesome video looking at strumming patterns that you'll likely find interesting: