This is not a complete answer but an addition. So among the other questions do ask or try to find out about past PhD students.

What is now their future as PhD holders. Are they in Academia? Are they hired in a company that you would like to work? You are about to walk a path that others also walked before you. It makes sense to see where they are now to get some indications on where this path may lead.

Some supervisors will even list past students in their websites.

The research should be extensive and to the level of the specific lab, professor, supervisor. Even within the same school of a university there can be quite a variation.

This is an addition to points mentioned in other answers. Among the other questions do ask or try to find out about past PhD students.

What is now their future as PhD holders. Are they in Academia? Are they hired in a company that you would like to work? Is it a good company? Did their PhD helped them in their career and was it required?

You are about to walk a path that others also walked before you. It makes sense to see where they are now to get some indications on where this path may lead.

Some supervisors will even list past students in their websites.

The research should be extensive and to the level of the specific lab, professor, supervisor. Even within the same school of a university there can be quite a variation.

This is not a complete answer but an addition. So among the other questions do ask or try to find out about past PhD students.

What is now their future as PhD holders. Are they in Academia? Are they hired in a company that you would like to work? You are about to walk a path that others also walked before you. It makes sense to see where they are now to get some indications on where this path may lead.

Some supervisors will even list past students in their websites.

This is not a complete answer but an addition. So among the other questions do ask or try to find out about past PhD students.

What is now their future as PhD holders. Are they in Academia? Are they hired in a company that you would like to work? You are about to walk a path that others also walked before you. It makes sense to see where they are now to get some indications on where this path may lead.

Some supervisors will even list past students in their websites.

The research should be extensive and to the level of the specific lab, professor, supervisor. Even within the same school of a university there can be quite a variation.