Daily driver running lean

[brainbug007]

My daily driver (renault scenic mk1) seems to be running quite lean at the moment. It really struggles if the revs fall too low and to pickup any speed in 4th/5th gear. I've checked the plugs and they've got the white crud on them instead of having a sort of brown color to them. It's had new head / intake gaskets recently and I've checked all the seals from the air filter to head with easy start and they all seem ok. This leads me to believe either the fuel supply isn't adequate (blockage?) or a sensor for the ecu is knackered. Is there any way to narrow down the problem without having to connect the ecu to a computer?