Three things to watch for Thursday night in the San Francisco 49ers' 2013 exhibition opener against the Denver Broncos at Candlestick Park (9 p.m. ET):1. Young receivers. A.J. Jenkins will be looking to shake Denver's defensive backs and perceptions of him as a disappointing 2012 first-round draft choice. Jenkins generally enjoyed a constructive offseason until a hamstring injury caused him to miss practice time this summer. Jenkins returned to practice this week and quickly made an impact with his speed.