How do I know if I need a coach?
The benefits of a coach are countless, but needing and wanting a coach are two different things. I would definitely contact a coach if you are new to the sport, especially if you don't have a background in one of the three athletic disciplines. If you are time crunched because you have a family, career, or other competing commitments, a coach will be a huge asset. Its good to seek out a coach if you don't have enough time to learn how to train safely. Over-training potentially leads to injuries before or during race day and under-training can lead to injuries on or after the race. Building base knowledge of techniques, gear, training schedules, etc. takes a lot of time. I recommend anyone just starting out, anybody who finds themselves plateauing & doesn't know what to do, or anyone who is crunched for time should hire a coach or at least join a group training program. Make sure you understand his or her coaching style and look for references and training credentials. Remember, racing and coaching are very different, so look at coaching experience and not just race experience.
The litmus test for needing a coach is simple. If you are satisfied with just finishing your races, stick with what you are doing. If you want to drop time, go for longer races, or regularly place in your AG, invest in a coach. Make sure they have SOME education (at least a BS) in something, preferably a biological science. Dont be distracted by all the certifications that they list. I have them, they are useless. Ask lots of questions. If the person cant make you understand their philosophy, find another coach.
Lisa has a great answer here, but I'd go further one step to say that anyone, at most any point in their career, can benefit from a good coach. Good luck!
The answer to this question is simple: if you are training and racing, you need a coach.
A coach is great for many reasons: technique, encouragement and motivation, rational thinking, good planning, just to name a few. I would definitely work it into your budget, whether that means joining the local club coached work-outs, attending small group sessions, or working one-on-one with a coach. You will find that you will enjoy the sport so much more knowing you have direction on how to successfully prepare.
To find a coach, I suggest looking at the Triathlon Mentor Coaches page (http://www.triathlonmentor.... 
).
All the best in your training!