How do I know the SMC has been reset? Is there a way to check?

You can Option Boot the Intel Mac and let it sit at the Apple boot picker menu. Perform the SMC Reset. The laptop should power off immediately. If it is a 2018-2020 Intel Mac with a T2 security chip, then use the second version of the SMC Reset involving multiple keys (read the Apple article @leroydouglas linked since the 2018+ T2 models have a different procedure for the SMC reset). On older Macs with a Magsafe charger, the charger's LED should go dark for a second, then cycle back on to green or amber.

Mafchew wrote:

I had to reset the SMC because my MacBook would not charge. I think that it worked but I want to make sure

MacBook Pro 16″, macOS 11.6

I can only assume this is an Intel Mac...

resetting the SMC How to reset the SMC of your Mac - Apple Support

You can not verify the action— if it worked it worked, no?

You can read more—

https://eclecticlight.co/2019/11/18/reset-and-maintain-your-smc-and-nvram/