How can I verify if transactions were merged?

Expand the Status/blue dot column.  In its default size you can only see the icons and have to know which each mean.  With it expanded a bit you will be able to see the text that goes with it.  For a match you will see New Match.  For a transaction that is new, but not matched you will just see New.

There isn't a preference to approve only the merged transactions.  There is a preference to confirm all downloaded transactions (I will point you to that option after explaining a bit more).

Note that you can click on that same status icon "New Match" and unmatch a transaction.  To manually match a transaction, you would make sure the amount in the manually entered transaction is the same as the downloaded one and then click on the "Uncleared" status icon and select the option to manually match.  And as a tip you can sort by the Status column (click on the Status column header) to get all the New/New Match transactions together to make sure that you see them all.  But be sure to click on the Date column to sort by date when you are finished reviewing them.

Also clicking on that status icon/text allows you to mark it reviewed, which will clear the status field.
The process should be that you review all your transactions after downloading to make sure the categories and payees are all correct, and then you would either mark the transactions reviewed or you could do a reconcile and after that the transactions will be marked reviewed.  Note is very important not to use automatic reconcile with the automatic transaction entry mode.  Reconciling doesn't care that your categories and payees are correct, only the amounts.  So, if you have them both on it will do a reconcile right after the downloading, and if that passes all those transactions will be marked reviewed and you won't have any status information to know what is new or not.

The other option is to turn off automatic transaction entry mode.  With this off you get a new tab at the bottom of the register, the Downloaded Transactions tab, and the transactions go there.  Then if you select a downloaded transaction from that list, it will show you that transaction in the register.  You then and hit Enter or the Accept button to have it actually enter it into the register.  There is also an Accept All, but be warned if you hit that all the transactions will be accepted into the register without you looking at them and the Status column will be blank.  This is the "old" method of accepting the transactions into the register oppose to the newer method that you are supposed to review transactions.  The big difference between the two is with the Downloaded Transactions tab you can only see what the resulting transaction is going to be one transaction at a time, and the filter/sorting is more limited than what is available in the register.

Here is the global preference:


You can override this in each account.  If you right click on an account name in the Account bar and select Edit/Delete account, it will take you to the account details. From there you can select the Online Services tab and select this link to change the setting to either follow the global setting or override it on or off.