Am1186...in the link below is what a route taxi will look like...
http://negril.com/nt/nt100616.htm

All legals taxis have a red plate which includes route, charter and tour taxis....a route taxi is decorated a bit more as you'll see in the above link.
With a group of 5 people, you would be better off chartering a taxi instead of trying to grab a route taxi. When you negotiate, most negotiations are based per person, so make sure it's understood you are paying per person or paying per taxi ride.