One way to test for creosote is to hold a glass of ice water in the stream of smoke coming out of your smoker. If you notice black specks on the glass after a minute or so then you don't have enough ventilation. Open the vents more to let more air travel through the smoker. If you have a vertical water smoker without vents then remove the lid for a minute to let the smoke escape. Once you have noticed the creosote it is time to stop adding wood to the fire. Reduce the smoke production, at least for a little while. At this point you might want to wrap the meat in foil and allow it to continue cooking without being exposed to more smoke.
Another way to test for creosote is by tasting the meat. Take a piece of the darkest meat along the surface and put it in your mouth. Let it sit on the tongue for a little bit. Does it taste bitter? Does your tongue feel a little numb? You will usually notice the numbness before you taste the bitterness.
Once the chemical reaction takes place the surface of smoked meats is pretty much ruined. The only hope you have left is to carve off the blackened edges and eat the interior of the meat. This is pretty much impossible with ribs, but can be done with brisket and pork roasts.