At least Toyota has a special hybrid battery test that only the dealership can do. If you service your car regularly at the dealership, the test is performed annually and in Finland, this extends the warranty 1 year at a time to 10 years (with 350 000 km max). I suspect the actual test is performed continuously on the car computer and the dealership just reads the test results stored in the car computer's memory.
Unfortunately, I don't know if Lincoln has a similar design than Toyota.
However, typically traditional (non-plug-in) hybrid vehicle batteries are oversized for the job they do. All they need to do is to help accelerate the car to max speed. For example, my 2016 RAV4 hybrid has a 1.6kWh battery pack. This is 5.8 MJ. In comparison, accelerating the 1700kg car to 120 km/h (the max speed allowed in Finland) takes about 1 MJ, and about only half of the acceleration energy comes from the battery, so 0.5 MJ required. So, the battery is over 11x bigger than it needs to be! Of course, the battery is shallow-cycled with only 40% of its capacity being used, but even then it is oversized by a huge margin. Not only that, but also the NiMH batteries last surprisingly long amounts of time due to shallow-cycling.
Additionally, since the batteries typically last for the lifetime of the car, you can almost certainly find a cheap junkyard battery instead of having to purchase a new battery.
So, I wouldn't worry about the battery. I would worry about buying outdated technology when purchasing a used hybrid, as a newer car very well can have better fuel efficiency.