What important information should you find out if the contact for a job advertisement is an employment agency?

If the employment agency will charge you a fee.

There are other considerations. Such as do they match the job to your skills? Do they understand what kind of skills you have to offer to prospective employers? Do they listen to you? Will they give the first job that comes along, or will they wait for a job that matches your skills? Also what is their reputation compared to other employment agencies? Last but not least research this agency. Ask around and see if anybody has worked with them. What is their opinion of this employment agency?

One last thing if they don't respect you, then find another agency who will respect you and give you the best results.