2.9 running rich???

I have a few questions regarding my 2.9. When at idle the RPM's seem to fluctuate up and down. I can't seem to find any vacuum leaks, so what else could be causing this? Also I've noticed a black sooty deposit on the inside of my tailpipe. Is this normal or could my engine be running rich, and if so how do I fix this. And last, How does one check for spark for each cylinder?
Thanks.