My check list:
Whom is the company? An agency looking for new talent, a modeling school looking for new "students", an online portfolio looking for new models, a company looking for new faces.
I then check to see if there is an entry fee for the model search and if so, what is the cost.
Then, work in perspective:
If it is an agency: there should be no cost to enter the search and you should contact the Better Business Bureau and/or state film commission to check them out.
If is a modeling school: Don't waste time at all, period
If it is an Online port: I wouldn't pay to enter at all, and if it was free, it would depend on what they were offering for their model search.
A company looking for new faces: Usually isn't any entry fee or if there is, it is very low. Ujena does have entry fees, yet is deemed by alot of folks in the industry as professional. There are many other companies that do models searches and don't charge entry fees: Texas Bikini Team, Hawaiian Tropics (the biggies there); and other small ones can include business that want models for promotions and addvertising that don't want to use an agency (night clubs, automotive business, local designers, ect.).
Basically, avoid modeling schools and any company that wants to charge large (or usually any) fees to enter.