Because they have different nikkud (vowel symbols): אַבָּא and אִמָּא. Of course you can't see that without the nikkud, and Hebrew is almost always written without it, so you have to learn the words and their pronunciation. When writing without nikkud, in many cases the i vowel is represented by the letter י (Yod), but there are exceptions and אמא is usually one of them (although it can also be seen as אימא sometimes).
Abba and amma are shared by Korean, Japanese, Hungarian and if I'm not mistaken Finnish as well. These languages were once theorized to have a common origin with Sumerian languages. Today only the Dravidian (Tamil) language maintains this relationship through its links to Indus Valley and Meluhha.
There are indications here (this is given as a hyperlink in the first Tips and Notes); you can for example add a virtual keyboard using the language manager of your operating system, add the DuoKeyboard plug-in for your navigator, or even directly use some online (or downloadable) virtual keyboard such as this one.