Catholics are taught to believe that Jesus is truly present under the appearance of bread and wine in the Eucharist. However, often Catholics may profess with their mouths but do not truly "believe" that Jesus is there.
In such cases, we can pray to the Holy Spirit, that we may be enlightened and see with the eyes of faith the reality of Jesus' presence in the Eucharist.
Here is a prayer adapted from Dom Prosper Gueranger in his Liturgical Year.