First find out what the Church teaches regarding it. If there is still ambiguity consider the following:
1. The devil never impels to virtue; nature usually inclines to the convenient and easy.
2. Does this thing I want to do conform to my state in life?
3. The devil agitates and fills with restlessness; God gives peace to the heart.
4. Humble obedience to our superiors.
5. What does my Spiritual Director say about this?
These were from a hand-out at a day of recollection. These are some things to consider when making a decision. Always pray when making a decision. If it is a small decision say a 'Hail Mary' if it is a life altering decision you may need to make a retreat to hear God's will for you and say many Rosaries.
The following link is to a chapter on discernment of spirits in Fr. Hardon's book on the Spiritual Exercises. It is much more detailed.