How do I tell which cards are "bombs" in draft?
First off, what is a bomb? According to a Limited Information article it is ... a powerful, game-ending, stabilizing card." though the author does admit that other people might have slightly different definitions. The MTG Salvation Wiki describes a bomb as "...a card, generally used in conjunction with Limited play, that always makes a large card impact on a game in which it is played."

From these definitions we can gather that 'bombs' are cards that when you play them have a high impact on the board, making it so that if you were behind you have now caught up, or even pulled ahead (stabilizing), and if unanswered will win you the game quickly (powerful, game-ending). In general I would say a bomb is anything that when played, regardless of board state, makes you go 'Oh crap' when played against you.

In general this means that a lot of bombs are creatures with high power and toughness (usually 5/5 or bigger), evasion or protection from removal is nice but not always required especially on ones with powerful 'enters the battlefield' triggers, speaking of which ETB triggers are also help make bombs because even if they have removal you still got some value out of the card. There can be other factors that that make creatures bombs but these are the big ones. 

While a lot of bombs are creatures it is possible to have non-creature bombs too, most Planeswalkers are bombs, for other non-creature non-Planeswalker cards generally you want things that will have a large impact on the board for example End Hostilities or any other board wipe, Overrun or Flying Crane Technique or other things that give your entire team a boost and/or evasion. For these though it is a bit harder to define exactly what makes a bomb since bomb creatures are designed primarily to attack, but non-creature bombs can do a wide variety of things.

As for Mystic of the Hidden Way, according to the definition I've been using it is not a bomb. While it does have evasion in its 'can't be blocked', it doesn't have very impressive stats at 3/2. This means it neither helps you stabilize your board if you are behind, since it can't block most cards without dying too, and while it will inevitably win the game for you if you keep attacking it will take a long time. This doesn't mean that it is a bad card, it just means it isn't a bomb.