Water Heaters - Dip Tube Replacement

Hi, I have a 1991 reliance 501 gas water heater that has worked excellent until the last month. The problem is water doesn't stay hot as long as it used too. I used to be able to have 2 people take 15 minute showers back to back, and it only go lukewarm in the last 5 minutes. Now after 10 minutes the water goes lukewarm and then cold. After reading several posts I'd like to think its the dip tube that has gone bad. How would I go about checking it to see if it needs replaced. If it is not the diptube what else should I check, please keep min mind it is gas fired.

Don't worry about the dip tube. 17 years is way beyond the life expectancy of your water heater. Replace it. If you don't, you will be having more problems in the near future than you can handle.