I would like to know if there is a way to check a hot water heater to see if it needs replacing. Our hot water heater is 20 years old, I was guaranteed this heater would last 20 years and my concern is I don't want the basement flooded again like 20 years ago.

The older hot water tanks have been known to last many years (20 or more years) but most new tanks will probably last 8 – 12 years with 15 years being the max.
Unfortunately, there is no way to tell how long a tank will keep on working unless there are some obvious signs like rust marks on the tank or browning around the flue hood. When appliances reach or exceed the expected age it’s sometimes better to be proactive rather than to be reactive. This will not only allow you to budget and to have the replacement done at your convenience, but will also save you the aggrevation and problems that will occur when a failure happens.