I agree with Mhiacel g's comments about the beggars having cellphones. I was on a bus once with a woman not much older than myself begging for cash rather unconvincingly for her toddler child and halfway on the ride she got a PM message on her cellphone (color screen, very fancy looking thing, must have cost her over 300 US dollars). Poor, yeah right.
You don't mention which city you're going. Resort hotels are like cruises where you eat at the restaurant (which tend to have dishes that Americans wouldn't find to be gross), swim in the pools, or bask in the sun in the beach. Everything is very Americanized and you get broadband internet and cable tv. Basically you could never leave the confinements of the hotel your whole trip and be happy, but never see the real Mexico.
Chances are you won't be in a situations where you'll drink tap water because you're going to a hotel and not living here as such. I agree on the suggs about eating in street joints.
Tacos and quesadillas are fully safe to eat because the heat used to cook em kills the bacteria (many street tacos stands have a separate guy who collects the cash and so the person that cooks the dishes doesn't get his hands dirty and thus you won't get sick unless your hands are dirty). Juice and fruits stands may or may not be safe, you'd have to take a good look at the hygiene used to prepare it. I've drunk freshly squeezed orange juice at these kinds of places dozens of times with no probs.
If there's fondas with comida corrida in the place you're going, I highly suggest them. These are the kinds of low cost restaurants real mexicans eat other than street taco stands. A full lunch from soup to dessert rarely costs more than 4 dollars per person and the food is usually very good. They usually have a rotating daily main course menu and the soups change each day, but the dish formulas are very similar: soup (many times chicken broth is commonly served)-rice or mexican spaghetti side dish-main course which is random-dessert (jello, custard or some other random small dessert).
I agree with avoiding tourist trapppy markets, go to the real bling-bling: the tianguis. Many tiangius on Sat mornings serve real mexican food. If they have beef soup I highly suggest it because it isn't commonly served in fondas or restaurants and it's clean food. I do warn that if you find cow intestines and such to be gross which might be served in the soup, you might not like beef soup but it does taste real good.
Depending on the time of year, tianguis sell pretty interesting fruits and veggies you won't find in the US. You could wash em at the hotel and eat em later. Some fruits you've never seen before that you might find are: xoconostle, mamey, zapote, higos (figs), granadas and stuff like mangos, oranges or papayas are usually available at these places dirty cheap. You can clean em by submerging em in water with a few drops of iodine for 10 minutes. Chances are you might encounter the cuitlacoche fungus which tastes real good in quesadillas or in egg omeletes.