How do you know this, now that LN is on the main network?

While not quite as count number of transactions in today's blocks simple, it's still quite easy to work out the transaction volume in the LN if you control some of the more popular channels.

(Remember, we're counting the number of transactions, not the amount of Bitcoin sent)

How do you measure this?

Adoption rate is a measure of how many people use bitcoin to buy goods and services.

It's completely trivial to find out by nature of how few stores actually accept Bitcoin.

How do you know this?

A) Basic statistics, the odds of whales buying/selling, crushing some margin traders, only to sell/buy leaving the price at the same point as before, repeatedly, are astronomically small.

B) Financial institutions don't refuse to do audits (ones they promised), unless something is seriously wrong. And when said financial institution(s) have a history of pulling shady, barely-legal, financial chicanery...