"I am seeing an egg."
Translation:Tha mi a' faicinn ugh.
You need to learn it by heart on a word-by-word basis. You can always check it in a dictionary.
As for any rules – there are some (like most words ending in -ag are feminine), but I don’t know any comprehensive list of those and there are always some exceptions. You’ll start noticing some patterns yourself when you learn enough words.