Its not that difficult of a fence to define.
You are still on the fence when you stop doing something about it. If you are worried you are still on the fence and are moving towards getting off the fence, you aren't on the fence. If in the back of your mind you are asking, "Am I doing enough" and you go out and do more, you aren't on the fence.
If in the back of your mind you ask "Am I doing enough" and go back to watching TV for 50hours a week, then you are probably still on the fence.
But worrying about being on the fence should not be your driving force for getting off of it. You love God, you love Jesus and you (at least I) want to show others that love. That drives me. How do I show my Lords love today? Somedays I don't think about the question and somedays I have to pray about it.
The whole passage we are referring to is interesting. It also says, its better to be Hot or Cold then lukewarm. To me that means, its better to make a decision, and give your all out for God or be all out against God then to say one or the other and do nothing about it.
Even though we may not like it, or understand why, DV and Eon etc, all made a decision to be Cold to God. And God respects their decision. And even moreso, God respects the fact they are here willing to share and debate and inform. Because God can use the active, even those who are against him. God can not use the inactive, those that sit and watch TV 50 hours a day is a dead branch. Better they be pruned, or spit from his mouth.
There is another thread about DV is a tool of God, or something to that effect. How true. Its better that DV be active here, and as active as he is. He is more like the servant who was entrusted with 10 talents in the parable of the talents. His work (whether he likes it or not, agrees with it or not, accepts it or not) is generating a return for Gods glory. Compared to the servant who was given 1 talent and buried it.