Odds of Hanging On
Working as a full-time, successful sketch artist can be a tough gig, and only a small percentage of people who want to do it are likely going to keep doing it, either by choice, or...not so much by choice. Competition for forensics contracts will be high, giving above-and-beyond types the greatest edge.
Did you go to college? Practice identification sketching? Get those optional certifications? These are the sort of questions you can expect a potential employer (or that little nagging voice in the back of your head) to ask.
You're also going to need to be persistent and hungry. You can't just sit in the park with pencil and paper and expect people to randomly begin lining up for $100 portraits. Artists who learn to market themselves, make connections, and keep clients will always have a better shot at staying in the game.