how to tell the difference?

[krstkim]

How do you tell the difference between:
じ (ji) and ぢ (ji) &
ず (zu) and づ (zu)?

How do you know when to use which (spelling)? Do you just have to remember which one a word is spelled with?
My apologies if this question has already been asked.

[QuackingShoe]

You just remember. It also usually doesn't matter, since those things are usually covered up with kanji. In by far the largest number of cases, じ and ず are used.
In the cases where ぢ and づ are used, it's typically because of sound changes due to kanji compounds (typically using kun readings?) or syllable repetition. So if you know the normal readings of the characters being combined, you should know what to use.
For example, 鼻血, nosebleed, is written はなぢ (rarely はなじ), because it's a combination of the words はな and ち. And as a random づ example, 詰 is read つめ, so 缶詰 is かんづめ. Also a VERY common (and visible, because the part that changes is often written in kana) example is when 付く(つく) is directly attached to certain other words, most prominently 気. It becomes 気づく.
An example of repetition would be 続く (つづく).

Don't worry about it. Ultimately, it's one of the least of your worries. It's not any more complicated than は、へ、を or the whole おお/おう, ええ/えい thing.

[jkid]

You just have to remember when to use which. Although,　these forms of zu and su are uncommon and are used mostly to indicate a voiced consonant in the middle of a compound word. They're never used at the beginning of a word.

For example "tsuzuku" or "to continue" is written as つづく this feature of Japanese morphology is called "Rendaku". If you're interested you can read more about it at:
http://en.wikipedia.org/wiki/Rendaku

[krstkim]

Thanks for the replies. I'm a beginner and only know hiragana as of now, so I don't know if I understand the whole thing about kanji (kanji still boggles me. I don't understand it at all.) as of now, but it will probably come easily to me as I learn more Japanese. I do, however, think I understand how the ji(s) and zu(s) work now, so thanks again for the replies.