Azure SQL Database finding latest available restore point
You can use Get-AzureRmSqlDatabaseRestorePoints to get the earliest possible restore point for the db.  The output from this command is below:  

ResourceGroupName        : 
ServerName               : 
DatabaseName             : testdb
Location                 : East US
RestorePointType         : CONTINUOUS
RestorePointCreationDate : 
EarliestRestoreDate      : 2/23/2019 1:53:30 AM
RestorePointLabel        : 


Unfortunately, this command does not give you the latest restore point.

According to Microsoft the transaction log backups generally occur every 5 - 10 minutes.  I attempted to do a test restore with the recovery point 2 minutes behind the current time and got the following error:


  Choose a restore point between earliest restore point and latest backup time which is 6 minute before current time.