After re-assembly, iMac 27" fans blowing at full speed all the time
Yes, there is a 3rd party app called Mac's Fan Control which allows you to adjust the speed of your fans. It gives you the current speed of your fans and has different options for setting the speed. You can set a variable speed which forces the fans to go faster as temperature goes up - as they are supposed to do. 

there is also an app Temperature Gauge 4.1 that lets you monitor the temperature of different areas of your Mac and the fan speeds.