Scroll down a bit in this article. There's a list of what each of the available keys are required to provide. A "key" in this context is basically a notch in a certain location, which then defines the meaning of the various pins of the connector. Some devices have multiple keys, as some of the specifications have a common subset. Like key A+E is common, because E provides almost everything that A does, so a device that only requires the common interfaces can work in both. Cars that rely on one of the exclusive interfaces will have the specific key of course. This A+E communication is often used for WiFi cards.

Sockets always only have one key though, for obvious reasons.

Edit: correction/clarification

The most reliable way I know is to seek documentation for the board. It's up there with PCI lanes in that the board designer will know what has been configured to work with that physical connector. This kind of info is definitely part of your motherboard documentation.

I'm not familiar with dmidecode so unfortunately I cannot comment on that.

Is there any way to find out whether it supports SSD storage?

Have a look at the notch in the slot. SSDs will have either M key, B+M key, or very rarely just B key.

WiFi modules are A, E, or A+E