What are the signs of battery swelling?

I've been trying to find information on it so I can watch out for my vita, but I can't seem to find a lot of answers besides like... Psp battery swelling stuff. Like so much psp battery swelling stuff.

Not a lot comes up specifically for vita. And since we can't exactly just pop the back off without some extra tools from games websites it's a little frustrating.

Although one symptom seems to be the console taking too long to charge or not retaining its charge (in a dramatic way rather than through natural degrading due to use and age.)

I feel stupid for asking this but would it show through the exterior?

I don't *think* mine is doing any of that (touch wood) but knowing the signs would be helpful. I had accidentally left it at 0% for around a year (it's since fully charged, turned on fine, retains the charge normally, the back isn't warped in the slightest, temperature is normal and everything is displaying normally) and I hadn't realised it so just trying to cover my bases and be safe.

Thanks for anyone's time.