How do i know that my Garmin Montana 700i is connected to the InReach satellites?

I might misunderstand the question. There is no "connected to the Iridium satellites" state. Iridium devices do not even attempt to connect to the Iridium satellites UNTIL they attempt to send something. "Something" might be a message, a sent track point, or an attempt to initiate an active message check. 

When the iR modem in the device is on, you will see an icon in the status bar. On the 7x0i, this is a curved, upward pointing arrow. 

Status icons are described here: https://www8.garmin.com/manuals/webhelp/GUID-20A50E5B-3DFB-42C7-A3B4-8D55519B532C/EN-US/GUID-2B584F4D-BC02-431E-BCCB-C945CC377343.html

Note that the presence of the iR modem icon does not mean that the device succeeded in contacting an iR satellite. It means that the device is trying - nothing more.

Thank you. That is very helpful.