Theoretical writes/sec limit
One way to do this will be to monitor the writes/sec for weeks or a month and then come up with a number based on what the max values that you are seeing.  Obviously this is a rough guide, and as your workload changes you would have to adjust the threshold as well.

You could also use disk benchmarking tools to test the throughput of your disk subsystem.  diskspd is something which we use.  Please also note that writes/sec is dependant on the block size that is used.  Based on what the drive is used for(data or transaction log)  and also based on the type of workload, SQL Server could use different.  You can read about SQL IO sizes here.  Obviously if you run this on a production system, you might have a performance impact.

A better performance counter to alert on would be sec/Write or sec/Read.  You can read about it here and here