“How can we be sure our motive for witnessing is right, so we won’t be ineffective?”
Ask God to search out your motives. However, even if your heart is not in the right place (I’m not talking about sin, but that you are going because of a sense of guilt or obligation), you should still go. If you were rescued from a burning building by a fireman who left the firehouse because he felt guilty not coming to the fire, as far as you are concerned, his motive for rescuing you is irrelevant. All that matters is that he did. So don’t get hung up on why you reach out to the lost, just do it, while there is still time. The quality is in the seed, not in the sower. This gives great consolation to those of us who feel we lack ability.
However, some who are worried about motive may lack motivation themselves. Love for God and love for the lost is all we need. Jesus told us to go, and gave us the Holy Spirit to help us. So if you are waiting for the “prompting” of the Holy Spirit to witness, just ask yourself if the people you are waiting to witness to fit the category Jesus mentioned in Mark 16:15: “Go into all the world and preach the gospel to every [person].” The word “every” puts them in the “need to hear the gospel” category. If you were sitting in one of the Titanic’s lifeboats, with plenty of room onboard, would you look to the captain for his approval before you reached out to each drowning person, when he has already commanded you to reach everyone you can? Of course not. A good rule of thumb is: if they are breathing, they need to hear the gospel. Regardless of motive, the important thing is that Christ is preached.