How to identify your weaknesses?

If you post the game here, I imagine that there will be at least a half a dozen people who will be happy to come forward and tell you that you need to do more work on your tactical ability.

As kindaspongey said, most will say work on tactics at your level. Once you reach a certain level - which is different for everyone - and you've analyzed enough with and without engines, an engine can act as a rough guide if you know how to interpret it's raw variations. Basically, the engine will tell you what you should have done, but it's up to you to see why you should have done so.

move for move and make thorough notes as you do so
even at my noobie level the times i started doing this seriously, as opposed to a quick runthrough with the engine, i began.to see some repeating patterns
one of the easiest ones i noticed had to do with time management
in my case, trying to open games solely on "opening principles" was getting me in serious time trouble in the first 10 moves
it was recommended to learn a few ideas from the italian game- not delve deep into theory but just a few ideas- especially concerning my light square bishop as white- and couple this with the opening principles
this has helped quite a bit as i have more time to work with and it doesnt quite feel like reinventing the wheel every game
another issue i was having was not pushing d4- for example- after my 1. e4- when my opponent did not challenge the center
along with self-analysis i would suggest posting your games on the forums or, better yet, get a coach to go over your games with you
a good coach can really focus your attention on specific areas you may not even be conscious of
one of the first things my coach did in our lessons was to adapt my "tactics training"- sure i still spend time on the tactics trainer but it was recommended to pay more attention to (emmanuel nieman has a book called "chess antennae") specific "signals" which will alert me to the opportunity to find tactics in a game

If you post the game here, I imagine that there will be at least a half a dozen people who will be happy to come forward and tell you that you need to do more work on your tactical ability.
LOL. So true, and at least 1/12 people who will paste a list of book suggestions that should help you.

one of the easiest ones i noticed had to do with time management
in my case, trying to open games solely on "opening principles" was getting me in serious time trouble in the first 10 moves
it was recommended to learn a few ideas from the italian game- not delve deep into theory but just a few ideas- especially concerning my light square bishop as white- and couple this with the opening principles
this has helped quite a bit as i have more time to work with and it doesnt quite feel like reinventing the wheel every game
With increasingly faster time controls your comment does make a case for knowing your openings really well in order to save time for a later critical position.