My wage is changing from hourly to Salary... ?

What is reasonable salary for a project coordinator in the construction field? I took my current position at a lower wage for 90 days. The company had bad luck with prior applicants so they wanted a trial period. 90 days is coming soon but I have never worked for Salary pay before. How do I know they are paying me what I'm worth and how do I protect myself from being overworked and underpaid?