how would I make sure?
Unfortunately, the only way to be sure you've paid all taxes due, is to do a rough run of your tax return.
Or you can do a very rough run of the numbers and file an extension and pay in enough to cover any imaginary shortfall. Then you'll get it as a refund when you actually file.
The extension is only an extension to file. You have to pay in all owed taxes anyway.
When I was a young couple, the withholding on my check always netted me a refund, so I got lazy about filing on time.