How do I find out whether the 3.5mm audio jack on the side of my MacBook Pro (2.9 GHz Dual-Core Intel Core i5, early 2015) allows audio input from a headset microphone. (I need a headset for Zoom).
In answering the more general question on headsets that will work on your MacBook Pro with Zoom I suggest considering Bluetooth and USB headsets in addition to headsets that connect by a TRRS 1/8" phone jack.
USB and Bluetooth headsets will be "future proof" since the TRRS phone jack is disappearing from new hardware. Another "future proof" option is Apple's USB-C adapter to go with any headset that uses the TRRS 1/8" phone jack. https://www.apple.com/shop/product/MU7E2AM/A/usb-c-to-35-mm-headphone-jack-adapter
Apple produced this USB-C adapter for their iPad Pro but it works on many other devices, including those not made by Apple.
I don't want to talk you out of a headset with a 1/8" phone jack. I just want you to know that if you can't find a headset with a phone jack that fits your needs then there's USB and Bluetooth headsets as options.
All PowerBook and MacBook models support TRRS style audio input. Check with the vendor of your headset if you want to use any buttons or volume control if you have questions. Apple branded headphones work well, as do many third party options.
Yes, it does support a combination of headphones and mic input on that jack (limited to Apple's spec) as indicated by the specs here: https://support.apple.com/kb/sp719