Best way is to check a dictionary, the entry will specify if it's an い-adjective or a な-adjective. That being said, if its last character ISN'T い, it's definitely a な-adjective! There are a few な-adjectives that end in い (like きれい[な]), but they're few and far between.
only the な adjectives will loose the な suffix if is not immediately followed by the noun? that won't happen to い adjectives?
In terms of basic construction with adjective + noun vs "noun is adjective," yes, that's right! If you're just starting to work with adjectives, check out the "Japanese Lessons" under the "Resources" tab on here, they might be helpful for you!
