I live in Upper Marlbroro and even the 20774 area is fairly large. So there isn't a straight foward answer to your question. Some parts closer to Forrestville and Lanham are a bit sketchy but there are areas that are not so I will agree with Kathy and look at the crime reports.
Check the crime with the police department- they can advise you on these matters since they deal with this on a day to day basis. Are you looking to buy or rent in Upper marlboro? I can help you with both in Uper Marlboro. I have been in the business 37 years and am an expert in the Upper Marlboro area! Call me at 301-785-1995l