If you have a sprain or any injury to your ankle, the first thing to test is whether you can bear weight. If you can, it’s probably not that serious. That’s a crude test, but it’s a very important one. If you have severe pain when you put weight on it then you’ve probably done something serious and at that point, I would highly recommend getting it checked out. The most painless, sensitive method is an MRI scan. If you go to A&E, they will do an X-ray for you, but X-rays look at bones for fractures. Occasionally there’s a fracture, but it’s extremely unusual to have massive fractures in sports injury.
The other mode of investigation is an ultrasound scan. But the trouble there is, if you have a sprained ankle, someone prodding it with an ultrasound probe is not ideal. The best way is to stick your leg into an MRI scanner. It will tell you what’s what.