The choice is our how

    "... Take your everyday, ordinary life—your sleeping, eating, going-to-work, and walking-around life—and place it before God as an offering. Embracing what God does for you is the best thing you can do for him. Don’t become so well-adjusted to your culture that you fit into it without even thinking. Instead, fix your attention on God. You’ll be changed from the inside out. Readily recognize what he wants from you, and quickly respond to it. Unlike the culture around you, always dragging you down to its level of immaturity, God brings the best out of you, develops well-formed maturity in you." [Romans 12:1-2, MSG]

If you look at that you can translate that into - '...be not conformed to the world (of) but be transformed by the word (in)' - at least that's one interpretation. Another way of looking at this is - if we aren't 'in' the world then we have little way to transform, bring a different perspective. But if all we are is 'of' then where's our witness? What are we providing as an alternative for others?

Reading further in Romans 12 it says:

   "And do not be conformed to this world [any longer with its superficial values and customs], but be transformed and progressively changed [as you mature spiritually] by the renewing of your mind [focusing on godly values and ethical attitudes], so that you may prove [for yourselves] what the will of God is, that which is good and acceptable and perfect [in His plan and purpose for you].    

                ...

I say to everyone of you not to think more highly of himself [and of his importance and ability] than he ought to think; but to think so as to have sound judgment, as God has apportioned to each a degree of faith [and a purpose designed for service]." [v 2-3]

Personally, I can't think of a better description and definition of the difference (and basis of personal judgment) between being IN and OF  the world. There are clues of do's and don't's populating the first 8 verses of Romans 12. Here are some more standards in which to judge yourself.

  "For just as in one [physical] body we have many parts, and these parts do not all have the same function or special use, so we, who are many, are [nevertheless just] one body in Christ, and individually [we are] parts one of another [mutually dependent on each other].  Since we have gifts that differ according to the grace given to us, each of us is to use them accordingly: if [someone has the gift of] prophecy, [let him speak a new message from God to His people] in proportion to the faith possessed; if service, in the act of serving; or he who teaches, in the act of teaching; or he who encourages, in the act of encouragement; he who gives, with generosity; he who leads, with diligence; he who shows mercy [in caring for others], with cheerfulness.' [v. 4-8]

Never say you have no way of determining if you are in or of. This outlines it perfectly. And, if you want further support of these scriptures, turn to 1 Corinthians 12 - the complete chapter, but especially verses 12-31. 

The choice is always our 'how' which comes after we choose if we will act 'in' or 'of'.