Depending on which dictionary you use, it will tell you if a word is usually written using Kana alone. For example,
Find words - Denshi Jisho
WWWJDIC does the same.
Apart from that I'm not sure if there's a surefire way to know. The best thing is probably exposure. The lack of Kanji mostly applies to very common words (such as これ、よろしくお願いします, etc). Since these words a very common you'll see them all the time and automatically realize that the original Kanji are not used. Also, if the word is common and the Kanji is rather difficult it's also likely that it's not being used. I wouldn't worry about it too much, you'll pick it up naturally. Also, if you type in Japanese on your PC/Mac it will most likely suggest the correct writing as a first choice. If I write よろしく、I won't get 宜しく if I don't explicitly go through the list and choose it.