What can you expect from conference interpreters? How to hire a good interpreter?

The interpreting profession is not regulated in Poland. Anyone can become an interpreter or claim to be one. You may therefore encounter false promises. On the other hand, finding good interpreters is feasible, provided you consider several aspects.
Look for interpreting that delivers precision and acceptable form, i.e. quality
Desired quality will be provided by skilled interpreters, who work within their area of competence, under appropriate working conditions. You can choose to trust this website or form your own opinion. It may be worthwhile to find out how much the interpreters know about subjects important for you, possibly listen to their delivery. If you can't - go with their experience. “Recommendations” are nice to have but they are not necessarily a sound measure of quality of work.
Look for experience
This is reflected by the number of days worked and the range of subjects covered. The volume of work done is a measure of professional success, which in turn implies good quality. The range of experience informs about areas of competence.
Expect good preparation
Good interpreters will readily prepare for your event if you inform them of the agenda and/or provide materials. However, bear in mind that pure knowledge of vocabulary is not enough in the area of medicine. Here, understanding of mechanisms, processes and phenomena is indispensable. This is where appropriate knowledge and experience are invaluable.
Expect commitment to high quality
A good interpreter works not only for the money but also for the satisfaction that comes from a job well done. Given the opportunity interpreters will be happy to discuss details of your event and the experienced ones will be able to give advice.

Expect a team of interpreters to populate your booth
(see under "Interpretation Types").  An interpreter who works single-handed for hours cannot maintain a high quality of interpretation, regardless of skill and knowledge. The limiting factor is human brain physiology. Hence, it is best to reject offers involving single-handed simultaneous interpreting.