How to find the current Azure RM Subscription
Now you can use the new Az commands
Get-AzContext
Get-AzureRmContext
gives you info about the selected subscription, default storage account, etc.
(Get-AzureRmContext).Subscription
gives you the current subscription.
You can also use Get-AzureRmSubscription
$sub = Get-AzureRmSubscription
$sub.SubscriptionId
This will give you the SubscriptionId