Well, if you have been dilligent with you oil changes, and used a good quality oil, you shouldn't need a flush. If you are really concerned, run a few oil changes with mobil1 synthetic and change every 3000 miles for 3 cycles. If you notice the oil is very dirty, you know it worked. Mobil1 Synthetic has very high detergents and will work better than any "flush" product out there. If you are afraid of synthetic (you shouldn't be), then try replacing 1 quart of your dino oil with a quart of Marvel Mystery Oil, which is high detergent also...will do the same in 3000 miles.

Generally though, if you don't notice your oil being very dirty when you change it (like really bad), you probably don't have much sludge in the engine. Do you notice anything that makes you want to flush? stuck lifter?