How to see the hearts in minecraft realms?

If you want information about players in the tab menu, you need scoreboards. Minecraft has some simple commands to do this, but you need operator permissions.

/scoreboard objectives add Hearts health will create an objective called "Hearts" based on the amount of health a player has.

/scoreboard objectives setdisplay list Hearts will display the objective "Hearts" on the tab menu, resulting in:

Past a certain point, this will turn into a number value, but that will only happen at around 20/25 hearts, which is unlikely in standard gameplay.

Note: This may appear as 0 until a player's health changes for the first time - afterwards it will operate normally.

You can read up further at the wiki.