It's not really possible from a distance to tell the difference between an L-17A, L-17B, or L-17C and a Navion A. Most of them have been modified so much that you really have to look at the serial number to know. Generally if you see a Navion with a big military pitot tube in the left wingtip, or one with a fuel filler cap on the left side of the fuselage, that's an L-17 (I think those are L-17B features, though).
Speaking of serial numbers, REAL L-17s have 47-xxxx or 48-xxxx serial numbers, just like any other USAAF or USAF aircraft. Civil Navions trying to pass themselves off as military birds display their NAA or Ryan serial numbers (my old airplane was NAV-4-1713, for example) as a substitute and are a dead giveaway.
The website
www.l-17.org has a nice lowdown on the L-17 if you're interested. The webmaster is the owner of the my family's former L-17B, which I grew up flying for 20-some years, 48-1007/N77BH.