can you make a call on wifi even if you don't have a cellular plan?

Yes. If your paired iPhone carrier supports Wi-Fi calling and the option has been turned on.

click here ➜ Use your Apple Watch without your iPhone nearby - Apple Support

and here ➜ Make phone calls on Apple Watch - Make Calls over Wi-Fi - Apple Support

Your Apple Watch can make phone calls without a cellular plan if your iPhone is within Bluetooth range and has a cellular connection or Wi-Fi calling enabled.

With watchOS 10, your Apple Watch can make FaceTime Audio calls to other Apple devices as long as it has a Wi-Fi connection or is connected to your iPhone that has a wireless connection. You can learn more here: Make and receive FaceTime audio calls in Messages on Apple Watch - Apple Support

Your question implies a non-cellular Apple Watch, or a cellular model without a service plan. If you are at home you would probably be within range to the paired iPhone, so a call on the watch would be routed to the iPhone and from there by whatever routing the iPhone selected - either cellular or WiFi calling (if available). If your iPhone was not available nearby the notes in Apple's support document

Use your Apple Watch without your iPhone nearby - Apple Support

suggest it should be capable of calling if everything is correctly configured.

The only way to know for sure is to test it for yourself.