1. hard starting, no starting, rough idle and poor acceleration - in the easy cases. Rust at the neck of the fuel tank or can where the cap threads in. Slime, rust or stickiness on the gas cap undersides
2. It will leave a very thick, grungy, sticky residue everywhere it comes in contact with anything...it will dissolve non-ethanol resistant hose, plastic tank, plastic carb piece or jets, it will bind valves, pins, springs and linkage
3. You can often drain the fuel tank and let it sit/settle in a clear container...within a short period of time you will see it start to settle out (not clear like water, but different layers, still), if you see particles in the container - it has started. If it smells foul, it has started.
4. Complete rebuild or replacement of the fuel line, up to and including the carb or injectors, which ever is most economical/practical. Run the engine at least weekly long enough for at least a fuel bowl full of fuel to run through, if it is going to be stored, top up the tank, add additive/stabilizer, turn off the fuel to the carb and then run the carb dry (engine will quit on it's own)
If it has only been a few weeks, you may be able to get by with fresh gas, carb cleaner, and some reputable additive. Current thinking is that the separation can occur in as little as 3 or 4 weeks, unless you have added stabilizer and kept the tank full.
