I don't know how confident you are with Japanese writing, but the way I tell the difference is based on it.
- Na-adjectives usually end in -i when i is the last mora in the reading of the kanji. E.g. 有名/yūmei/'famous' - as you can see, no hiragana.
- Loanword adjectives are usually na-adjectives, so if one of those ends in -i, it's probably a na-adjective. E.g. ドライ/dorai/'dry' - it's in katakana because it's a loanword from English, so it's a na-adjective.
- I-adjectives add an -i (or some hiragana ending in -i like -shii) to the reading of the kanji, as okurigana. E.g. 楽しい/tanoshii/fun - the hiragana is added to the kanji.
So in a nutshell, for na-adjectives, the final -i is part of the kanji, and for i-adjectives, it's added to the kanji.
There are some exceptions to this, of course. The grammar's a little complicated, but a verb that ends in the mora -u
conjugates to ending in -i
in the masu-stem form, and that form can then sometimes be used as a na-adjective, e.g. kirai
/'hate', which is a form of kirau
/"to hate" (although, like its opposite suki
/'like', which is a form of suku
/"to like", I think the adjective is more common than the verb). So that's written as 嫌い, which looks like an i-adjective even by the standards I said above, but it's a na-adjective. This only applies when the verb conjugated in the masu-stem
form ends in -i
, though. Verbs conjugated in other forms that end in -i
(e.g. the negative -nai
or the "want to do" -tai
) act like i-adjectives.
Then you've got words that are always in hiragana, rarely or never in kanji, which makes it a little more difficult to tell the difference, e.g. the suffix -mitai
/"similar to" (which is a na-adjective, but you'd have to look it up to know). In this particular case, it's also important to keep clear in your mind that this mitai
is different from the mitai
that means "want to see", which is an i-adjective.
If you're not ready for kanji yet, then I'd recommend just memorising any adjectives you come across in the meantime, but once you start learning kanji, it really becomes a lot easier to tell the difference. I find it easier just to memorise the negative than to try to memorise what type of adjective it is: You can easily figure out what type it is if you know the negative.
If you want a list of na-adjectives that end in -i
, you can search jisho.org for "*い #adj-na" (without the quotes).