How and when do I take God's word literally? Every time I talk to a non-believer, they reference several things that they think make their argument stronger. 

When I talk about what God says for us not and not to do, they bring up things like "God tells us not to shave our heads," to which I say that God was speaking to people like Moses and the principal of faith comes in; if you follow God, no matter what he says, then great things will happen. (The passages referenced are found in Corinthians, around chapter 11) 

How do I explain that God is often meteorically talented (if and only if I'm right here), and that sometimes we don't need to take EVERYTHING he says literally? 

How do I tell if he is telling something that should be taken word for word?
Basically if you are talking to a non-believer that is not a Jew - He is not under the Law of Moses before of after Christ Jesus. He could turn xtian and start to LITERALLY uphold the Law of Moses but that would curse him. Xtians are not under the Law of Moses.

Romans 3 and Galatians 3 explains that xtians are not under the law of Moses anymore:
1) everyone sinned under the Law of Moses
2) there is not even ONE righteous person

Usually God would count on that one righteous person to save mankind and avoid God's wrath. Thus Christ Jesus, to save mankind AND show how to UPHOLD the Law of Moses thru faith.

Romans 3:30 NET


  Since God is one, he will justify the circumcised by faith and the
  uncircumcised through faith


This is where your denomination or nondenominational of choice comes in: To show you how to UPHOLD the Law of Moses through FAITH IN CHRIST JESUS.

Titus 3:4 NET


  But “when the kindness of God our Savior and his love for mankind
  appeared, he saved us not by works of righteousness that we have done
  but on the basis of his mercy


thanks for reading