LineageOS that Support Volte?

Is there any way to tell if a LineageOS phone supports Volte?

You can check the sources to see if it supports it, but even then your carrier may not allow it or it may lack the required configs for your carrier.

Notably anything older than 2016, does not afaik, devices then used non-standard IMS stacks.

What exactly should I be looking for when I look at the source code?

Check for an ims.apk in the proprietary.txt files for your device.
And because some devices use a common device tree, find the command device tree
name in the lineage.dependencies file and check the proprietary.txt files in
that repo too.