Well, " ı" and "i", and "u" and "ü" sound different from each other, so that will be your first clue. Can you hear the difference between them? If you can't, you should practice doing that, so you can get better at it.
If you are trying to spell a word and just can't remember how it goes, remember the vowel harmony rules. u, o, a and ı go together and so do ü, ö, e, and i. So if you know there's an a in the word, and you don't know if the next vowel is an i or ı, it is most likely going to be an ı, unless the word is borrowed from another language or is for some reason irregular.
Does that help at all?