Elisabeth Elliot, missionary and widow of martyred missionary Jim Elliot, has written quite a bit about the topic of being called. In her short essay "But I Don't Feel Called" from her book All That Was Ever Ours..., she shares her own struggles in hearing God's call:
The word "missionary" never actually appears in the Bible. God calls us witnesses. A witness is someone who has seen something. We are all called to be witnesses for Christ. You are either a good one, or a bad one. But you are one.
Most of us know what we are called to. We just get stuck on the obedience part.
Did Jesus say that only certain people are called to be workers in the Kingdom?
To fulfill the Great Comission?
Did Jesus say you must go to seminary first? (not that this is bad)
Don't wait for a special sign.
Don't wait for the sky to part and angels to sing the Hallelujah Chorus.
Don't wait your whole life for the "right" time. Don't wait until you're "ready" (whatever that means). The sign may never come, it's potentially never the right time, and you will never be fully ready. It's kind of like parenthood, you decide to go for it, and then you find out the only way you can survive it is by the grace of God.
My two cents, for what it's worth. (not that you should listen to me) :D
*photo credit The Cloud Appreciation Society*