1) Make sure they have medical backup arrangements and that you're not the sole responsible person knowing the area with the group.
Also be sure that they have standards for any onsite drivers, that they have had a good reputation with the "team" of support folks, and that if you're in an area where "manana-think" is prevalent, the interfaces between this and the genial type-A behavior usually necessary to keep a tour group moving forward have been worked out and you don't have a driver who hangs out at home when you need him to get you to the airport (er....these days, maybe the train station) on time.
2) Find out how many people typically go and work out what the staff-to-client/participant ratio is. You can handle, say, 40-50 people in a tour bus by yourself but on a longer stint, everyone needs to be spelled and I wouldn't want to be in a group where the ratio was anything more than 20 adults-per-staff-member.
3) If you're going into areas where your institution regularly files for extra medical travel coverage for you (the areas where this is necessary are usually listed with your OSR and I believe come from the US Dept. of State) be sure the tour company is giving you similar coverage.
Check your own medical policies as well to be sure you're covered for any sites you might be going through on your way there--you may have your own itinerary if this is a place you often go for your own research, and be covered for those places, but the tour company may have another routing and if there are stopovers and you get sick at one of them, you don't want to be paying out-of-pocket because it's in a non-covered place.
4) You could also work out something of the per diem from the USDS website to get a sense of your probable expenses and how what they're paying you and what they're covering for you works out to your income level:
http://aoprals.state.gov/web920/per_diem_action.asp?MenuHide=1&CountryCode=0000 Again, that's probably moot if you know the area well from having visited it before, but I usually have my own "haunts" for my research sites where I stay and hang out, and they'd be much less expensive than anything I'd be expected to do if I were guiding a group there....so I'd want to factor in some extra personal expenses as well, probably.
5) I would usually work up an evening presentation before the day's tour, (I do Ppts but you could just do slides or even just a short talk) and have handouts prepared for the next day's activity in advance; some people take these very seriously and want to "study up."
Others are really there for the off-hand interest and they're the ones you want to watch out for; they're more likely to wander off, goof up your time tables by getting lost and not paying attention to schedules, find some item in a shop and spend hours bargaining for it, not have their luggage out in the hall the night before an early AM departure, etc.
Tour guiding is fun but it requires certain kinds of preparation and the ability to keep a very careful watch on all the participants at once. Your sense of enthusiasm, etc. is certainly a large part of it, but they're like kids on holiday (hmmm...in fact, they ARE usually just slightly older kids on holiday.....)
In other words, study any materials you have on cat-herding with a sub-study on flea-herding and you should be OK.
Have fun.
