For weekly iteration planning, at the start of a week, ask the team to figure out the *tasks* that need to be completed that week to complete the deliverable (functionality) due at the end of the week. Then, have them estimate the HOURS/DAYS required to complete the tasks. This weekly total you plot on a graph, along with a revised set of these estimates that get updated every morning throughout the week. This is called a "Sprint Burndown Chart" in Scrum terminology. You can forecast the completion date by analyzing the slope of the trend line.

If you notice that the line is going up (things got more complicated) or are flat (no progress being made) then you will know within 24 hours if the team is updating this daily. The date slippage will be highlighted IMMEDIATELY, and you can take immediate corrective action.