Yes, do check your spark plugs. My 1998 CRV became a gas thirsty beast last month. Other than that, everything else was normal - start up, power, engine smoothness, etc. No CEL. First thing I did was pull the spark plugs. All 4 were very very sooty, but no oily character. Surely, I said to myself, all 4 (original to the car) injectors can't go bad at the same time. The center electrodes still had sharp, squarish edges but the gaps all looked bigger than normal. Sure enough, they were at 0.052" with my feeler gauge. This, on new original Denso copper spark plugs I got from the dealer which I measured at the factory spec 0.043" before I installed them myself 6500 miles ago.
I cleaned them with a brass brush, and regapped to 0.043". As an experiment, I returned two of them to the engine and used new copper plugs for the other two cylinders. After 500 miles, I noted the gas mileage was back to normal. I pulled the plugs again, and saw the two new plugs still had white porcelain insulator! The other old two had grey (not tan) insulators. Because the engine had run rich, I changed the oil the next week. (It was almost due anyway.)
Checking the owner's manual, it recommends checking the plugs every 6000 miles. I had forgotten about this totally, because the previous 2 sets of plugs I had in there were long life platinums and I had gotten complacent.