Just use the "Receive" tab.
From there, Electrum will automatically select one of your unused receiving address each time you create an invoice (receive queue).
That way, you don't have to manually check the addresses if those are already used, because as the consensus here: it's not recommended to reuse addresses.
You can check if the address is already used before in Addresses tab too. There is a number of transactions in front of each address and there is also filters on top that can be applied to the list to only show addresses that have never been used, or addresses that have funds, etc.
I particularly dislike the Receive tab because it has caused a lot of confusion among newcomers with its "request payment" feature that expires.