Please excuse me if this is ignorant but if there are no blocks how does one know when a tx is "confirmed" ?

How do I know there is not another tx around the corner that has an even highter consensus value that some of the trusted nodes just haven't seen yet ?

Is there a specific time that a tx needs to be on the ledger for so it can be considered confirmed ?
Is there a certain threshhold of consensus value that a tx needs before it is actually added to the ledger at which point every future tx will be disregarded ?