How can I know if I am called to full-time service?

    Opinions vary on this. Some believe that God gives a special, direct message to those He calls. But most full-timers would agree that rather than a special message, several clearly definable factors are involved in discerning God's call. These are seen clearly in Paul's description of the requirements for elders in 1 Timothy 3:1-7:

Desire
    One must want to serve full-time (3:1; cf. 1 Peter 5:1-4).
 

Character
    One must demonstrate spiritual maturity and godly Christian behavior (3:1-6).
 

Capability
    Elders and those who lead and minister the Word must be able to teach the Word of God (3:2) and defend the faith (3:9). Usually pastors and teachers will have the gift of teaching. Other full-time ministries may not require these skills to the same degree, but would require other skills appropriate to the work.
 

Opportunity
    Although it is not stated, one must not go into full-time service if it means neglecting other, higher responsibilities (such as the responsibility to care for family, cf. 1 Timothy 5:8). If you are considering full-time service, you need to seek the counsel of other mature Christians, who can help you honestly assess yourself to see if you meet these qualifications. Keep in mind that growth toward meeting the qualifications is more important than reaching a specific level of expectation (see 1 Timothy 4:12-15, especially v. 15).
    In summary, God does not usually "call" full-timers by an audible voice or a super natural message (although there are some such cases in Scripture). However, because He knows our hearts, and because He is the one who controls our circumstances and gives us our skills and desires, He does provide a discernible call through desire, character, capabilities, and opportunities.