To know if the pie is set, the middle should be slightly firm to touch and should not wiggle when the pie plate has been gently shaken. You can also insert a knife into the middle of the pie. If it comes out clean, it is set. However, this could cause cracking in the pie filling as it cools.  You can also use an instant-read thermometer in the middle. The filling should be 175F. The filling will continue to set as it cools.