"Know ye not, that to whom ye yield yourselves servants to obey, his servants ye are to whom ye obey; whether of sin unto death, or of obedience unto righteousness?" Romans 6:16
Jesus said that no one could serve two masters- it was impossible because he would wind up loving one and hating the other. Trying to serve two masters is also illustrated in Revelation by the term lukewarm. It is impossible to be hot and cold at the same time. How do we know if we are trying to serve two masters? How do we know if we are trying to serve someone or something else other than the Lord? If we profess Christ, if we have truly had a born again experience, if we are endeavoring to serve the Lord, then are their times when we turn our attention to serving something else besides Him? How do we know?
The answer lies in the verse above. In this verse Paul was addressing the church, the believer, the Christians, those who profess Christ to help them to continue in their service to Christ. So, how do we know? "Know ye not, that to whom ye yield yourselves servants to obey, his servants ye are to whom ye obey?" In other words-- whatever you are yielding to becomes your master and you become its servant. If you aren't serving Christ at all, then you are a servant of sin and it is your master. If you are living for the Lord but in a moment of temptation you give in to sin, then at the moment you yield to the temptation, whatever it may be, you are serving it not Christ- you can't serve both.
Again, how do you know? In the moments that you yield to your flesh and give in to your desires putting them above the commands of God's word and His desires, you are serving something else besides Him. Serving Christ is not a one time thing. It is done day by day, moment by moment, and it is done by yielding to Him in everything- in every choice, in every decision, at each point of temptation. So the answer is found in the "yielding". Whatever you yield to is what you are serving.
Have a great day. Whatever you yield to is what you are serving, and you can't serve more than one master at a time. So who are you serving?