How can I check for activation lock before I purchase a used MAC?

There is no absolute means to confirm there is a lock on a used Mac and caveat emptor prevails. Buy it from the Apple Refurb site and there will be no NVRAM lock on the device. It will boot and present you with the normal setup options.

Even though it was written for iOS devices, most of the following User Tip is equally applicable to Macs:

The All Too Common SAD Reality of Buying a Used iPad/iPhone - Apple Community

I encourage you to read it.

The previous owner must properly prepare it for resale. If they don’t, then don’t buy it.

see > What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

I've seen so much grief here over used Macs said to be "ready for sale" that I cannot ever recommend buying used.

If you must do this, get a written guarantee stating you get all your money back if the computer is not properly prepared for sale and reuse. Even then, such docs are not very easy to enforce if the seller balks, and your resulting legal fees from attempting to get your money back could buy your a really expensive, top-of-line new Mac new.

Mac are in high demand worldwide and that fact has opened fertile ground for shady sellers ready to cheat over-eager buyers. Your safest action is to run away. 🏃🏼‍♂️