Reach in and touch his prick. If he's hot for you, it will already be hard. If he's nervous, it may still be bent up inside his shorts. Grab hold of his cock and lean up to his face and kiss him. See if you can feel a response down below.
Any movement is a good sign.
Now pull his cock out and see how it stands. If the pants are too tight, pull them down. Now hold his balls in your left hand and his cock in your right. Squeeze it gently down toward the bottom of the shaft and get ready to suck. Run your tongue over your lips to get them good and wet and look into your man's face.