The host research group
An « international star » at the head of a research group and a long list of publications do not necessarily guarantee a high quality doctoral position. At the customary interview, it’s important to visit the research unit and take the opportunity to talk to the other staff members.
The working environment also plays a big role. The best way of getting an idea of the atmosphere and quality of a place is to visit the institution and talk to the team and team leader.
In exchange for a seminar, the host institution may possibly pay your travel expenses (find out before you go).
Making contact with local young researcher associations and asking precise questions can be useful both for forming an opinion of a working team and for more practical aspects (current salaries, possible financing, working conditions, etc.).