Ribs are about the most "by feel" piece of meat most people smoke. There is not a good place to get a therm into the meat without hitting a bone or going to far into or through the meat. For baby back ribs most racks run approx. 5 hrs., for spare ribs and St. Louis cut ribs more like 6 hrs.
As Nickelmore said make sure you test the thermometer in boiling water. The factory thermometers installed in most smokers can be off by as much as 75 degrees!
OK... once your ribs are on the smoker leave them alone for 2 hrs. - no peeking! After 2 hrs. take a look and check for pull back, you want to see about 1/4" of pull back (meat pulling back from the cut end of bones). If you have 1/4" of pull back you are ready to foil, if you are foiling. If you are not foiling give them a light spritz of apple juice or whatever.
Wait another 1.5 hrs., if you have foiled them take out of foil and put back on grill of smoker. If you like to sauce your ribs now is the time I like to do it. If you didn't foil then just add sauce or spritz again.
Wait about 1 more hour then check for doneness. I use two methods to check. The bend test - pick up a slab of ribs with tongs about 1/3 of the way from the thicker end and hold level and elevated above grill, if they are done and tender you will see the strands of meat start to seperate right where the ribs bend downward. The other thing I do sometimes is to cut a bone of the end, it should but very easy and cleanly - plus this makes for a nice "cook snack".
These are some basic guidlines. Since everybody's smoker runs differant, and every piece of meat is differant you have to adjust times a tad based upon what you see and feel. Once you get a good feel for ribs they are easy to do, and you will be the BBQ champ in the eyes of friends and family!