The usage of falls is exclusive to the conditional 1 but wenn isn't. For this reason falls is always translated to if. Well, of course you can use synonyms like in case of as well.
Wenn, however, can be translated to if only in conditional sentences. In case of being used as temporal conjunction you should go with when.
Sag bitte Bescheid, wenn du fertig bist! - Le me know, when you're done.
In case of conditional 1 I can't think of any sentences in which it isn't appropriate to interchange falls and when. If you're unsure whether or not wenn is conditional you can try to replace it with falls. If falls fits wenn is conditional and you translate it with if. This works only for conditional 1 as Emanuel pointed out in his comment. Conditional 2 is only build with wenn.
Last but not least, wenn has a lot of different meanings. You can translate, for example, wenn auch with although, even though. Looking at the numerous definitions of if I'd say that there certainly are some cases in which if is translated to wenn although not being conditional. But this is not part of this question I guess.