In that way I won't get many tasks because most of the tasks already remains full. I do like to work in microworkers instead as they have some function to restrict further submission if the enrollment is over.You have to focus on task number of positions.
So for example if it is 25 positions only then no longer available frequency is high.
However if job is for 1000 positions and there is 450 performed task then risk will be decreased.
Also check if job is near to end means 1980/2015 means you have to avoid job to no end by job is no longer available.
There are a kind of job I have see this morning Power Job for 2 $ where fees are reduced to employer and workers are charged for it.