comparing contents of a PST vs. what's in Exchange / o365 office 365

I've moved a few clients from POP to office 365.  I'll import the existing PST in outlook from the POP account into the o365 exchange account.  

The system will say it's done importing, but other than spot checking - is that contact now in exchange?  is that item in the calendar? Is that mail folder there and does it have x emails in it - I can't really be sure that everything that was in the PST is now in o365.  And even if it's in outlook in the OST, I HAVE seen that something hasn't been pushed to o365 - you'll see it in outlook on the machine you imported the PST but if you go to OWA / another machine with that account, the item isn't there.

I've used beyond compare for years to compare folders on a hard drive / network.  Is there an equivalent app to compare a PST  / what's in outlook vs. o365?

As soon as you import a PST into the OST of o365, there's more in outlook, but over time (minutes? hours? days? weeks?) outlook pushes things up to o365, right? How do you know when it's done?  at the bottom it says folders are in sync, right?

thanks!
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Vasil Michev (MVP)Commented:
One way is to monitor the 'server-side' info and compare it to the client side. In Outlook, you can do this by checking the folders Properties -> Folder Size -> compare the values on the Local data and Server data tabs.

Similarly, you can get this info via PowerShell. Get-MailboxStatistics will show you the total mailbox size. Get-MailboxFolderStatistics will give you the number of items and size of folder and its subfolders.

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