Assuming you supply the right voltage to the house, other necessary things are that the frequency is right (usually 60 Hz for 120 volts and 50 hz for 240 volts) and also that the generator can supply enough power at that voltage.
On each appliance, you will normally see a plate that says how much power it uses.
Maybe it is a coffee maker and it says on the label that it uses 840 watts at 120 volts. This is all you need to know, but you could work out from this that the coffee maker uses (840 watts / 120 volts) = 7 amps.
So, if you work out all the power requirements of the things that could be used at once, you can just add up the powers in watts.
In most houses there are a lot of things that just use a small amount of power like 20 or 30 watts. Things like computer modems or radios are like this. You can guess a bit with those, but big items like room heaters or washing machines you would need to read the label and get it right.
Having done all that, you would probably double it so that the generator is not being heavily loaded all the time.
Another thing you need to know is how much the generator voltage changes when you draw more or less current from it. It should be a fairly constant voltage out (and the right voltage) or the lights in the house will be changing in brightness when you turn on the coffee maker for example.