How can I tell what the hundreds/thousands digits are in a players level?

Solution 1:

Based off this Reddit, it is the following:

Level |   Color 
---------------
100   |   Green
200   |   Teal
300   |   Purple
400   |   Red
500   |   Yellow

Repeating over and over. You get additional gems on your portrait and your portrait shape improves each cycle.

To give you an idea, this is what the borders look like:

_(Source)

According to the image source:

Your portrait badge art will become more intricate for every 25 levels you earn. Upon reaching level 100, your visible level will reset to 1 and you’ll don a badge that’s even more epic so that you can show off your dedication anywhere your portrait is visible.

I think the Wiki's description is better however:

• Player progression badge art will receive more decoration every 25 player levels.
• Every 100 player levels, the level displayed in the progression badge will reset, and the badge will upgrade with even more ornate artwork.
• The Basic Portrait Border will update every 100 levels with new art that matches the player’s progression badge.

Solution 2:

At the time of posting Timmy Jim's answer was correct. Based on feedback about how confusing determining a player's level was Blizzard made a UI change on May 16, 2017:

Player levels displayed in portraits have received visual updates, and should be easier to distinguish while using the progression portrait border.