The flow of what ? It depends in what context it is said.
The 'flow' is the act, rate or manner of a flowing substance ( usually a liquid )or the continuous progression of a certain phenomena.
' to check the flow' could mean to control ,restrain or slow the progress of a certain flow or just to examine the flow for accuracy, quality or progress.
*In oilfield for example a "flow check" is a test performed to ensure stable well conditions or the integrity of a plug, valve or flow-control device. In most cases, the flow check involves observing stable fluid levels or conditions for a prescribed period.