Learn Farsi: تو مکانیکی؟

It is attached to the end of the noun! The Farsi word for mechanic is مکانیک (mekânik). The ی (-i) at the end is a shortened verb ending (called an enclitic) that means "you are". So, مکانیکی (mekâniki) translates to "mechanic-you-are".

If I wanted to say "I am a mechanic," how would this sentence change?

You would change the pronoun to من (man) for "I", and use the matching verb ending ـم (-am). The sentence would become من مکانیکم (man mekânikam).

Why isn't there a word for "a" before mechanic?

In English, we say "Are you a mechanic?" but Farsi usually leaves out the word for "a" (یک, yek) when stating someone's profession. You simply state the profession and add the verb ending.

Is there a specific word here that makes this a question?

No. Just like in casual English ("You a mechanic?"), Farsi often turns statements into questions simply by raising the tone of your voice at the end of the sentence. In writing, only the question mark tells you it is a question!