When you are creating open batches in ShipStation, these are user-specific. Another user on your account will know what orders you have in your batches through a couple different means.
You are able to see the batch number for any orders currently in an open batch by looking in the Open Batch # column on the Orders tab. Hovering over the batch number will display the user's name with that batch. Batches you've created will have bold and blue text; batches other users have created will have black text. If it is your batch, you can click on the batch number to view all orders currently in that batch.
ShipStation helps to avoid duplicates by not allowing an order to be placed into two different batches at the same time. If you or another user already has an order in a batch, you'll see the following notification if you attempt to add that order to another batch...
You then get the option to exclude the order, or move it to the new batch.