Tough one to prove conclusively unless you can inject latency into one of the links.
That said, the page you need is:
Security & SD-WAN -> VPN Status, which shows the link being chosen for each flow - whether its the primary link, the only link, or its being chosen on basis of performance.
Then click on one of the peers on this table (which is on the above page):
And it will take you two a page showing all the latency, jitter and loss metrics for the links to/from that site. This should show that the MX is monitoring the links. As I said, causing a failover needs you to then be able to introduce latency, and then you can show that the chosen link changes from the VPN Status page.