When you are adding fuel, clearly display next to odometer whether the site thinks it is KM, or Miles you are entering - possibly have an option regardless of what generic format (us / uk / metric) you have.
Once you set the reporting units for your vehicle they never change. So even if you change the display units for the site by clicking 'US', 'Metric', or 'UK' in the upper left corner of any page--you will always report using the units you chose when you set up the car.
Unless you have multiple vehicles with different reporting units, I think you'll always know how you enter fuel-ups because it doesn't change.
I have a couple of vehicles, one of which is using Kilometers, the others use miles. As I'm based in the US, all of the vehicles are set to report in gallons and miles. With the van being in kilometers, Fuelly has the option to enter your odometer in whatever value you set it to, then report your data in again whatever value you set it to. In this case, I enter the odometer in Kilometers and have Fuelly track it in Miles. Works perfectly.
And even though I have different vehicles with different settings in my garage, I do not have issues with confusing them.