First, you need a certain level of mental quiet and stability. Spirit senses can be subtle, and if you are busy thinking about what you're going to say to someone at work tomorrow, you can easily miss the signal. So practice relaxing, calming, quieting, and remaining in that quiet for a period of time. In short, meditation.
Second, you need to be familiar with what Eugene Gendlin calls the felt sense. Try this: once you are calm and quiet, pay attention to the area of your heart and your belly and then think for a few moments about your father. Notice any bodily shifts. Next think about your mother. How does your body respond? Now think about Jesus. How about Lucifer? This should give you some sense of what the felt sense is about, but you still need to refine your ability to listen to it and to discern the nuances that it has to communicate. This is something you can work on in daily life by coming back to it regularly throughout the day and asking yourself questions about how you feel about whatever is going on. You can also practice writing out what your felt sense seems to be saying. It can sometimes be communicating rather specific messages that you don't recognize the detail involved into you try to put it into words.
Now when you evoke a spirit, pay attention to this felt sense. There is usually an unmistakable shift when the spirit arrives.