Check the RR Database for the area of interest. The Mode column will tell you what type of system that is and from that what type of scanner you'll need. FM or FMN will be analog. P25 will be digital.
For trunked systems, the column of interest is Type. Match your scanner with the system listed there. For some systems, they support both analog and digital users. The Mode column on the talkgroup will tell you what type that particular talkgroup is. A will be analog. D will be digital. T will be the newer Phase II digital type that only the PSR-800 and new Uniden scanners support. E or X indicates an encrypted talkgroup that no scanner will work on unless they somehow turn off encryption.
One thing to remember, a digital scanner is like an analog one on steroids. It will pick up all of the analog stuff plus the digital modes. To be safe (or future proof your purchase), get a digital one and you should have the best of both the analog and digital worlds.
Another thing to remember is some agencies use a mode that no current scanner will work on. This is in addition to the encrypted ones mentioned above. Some examples of these types are MotoTRBO, OpenSky, ProVoice, etc. The WIKI pages for each scanner will indicate what types of signals they work on and should help guide you.