No easy answers.

But you can start by comparing natural gas consumption during winter periods compared to summer periods. Presuming the hot water and cooking gas usage is similar year round, the delta will mostly be the furnace consumption.

Further math can normalize the furnace usage against 'heating degree days' during each heating season (or billing period). A colder winter will require more fuel, sometimes knowing how they correlate for your particular house can be useful, or not.

Furnace fan I don't worry about so much. In winter the electricity used to run the circulation fan effectively becomes heat into the house interior, so a bit of electric heat instead of a matching bit of natural gas heat. Any savings would be mostly the delta between gas and electric for the marginal cost of that energy,

Air Conditioning electricity cost, similar rough calculation comparing spring/fall and peak summer electricity consumption. Exclude the dark winter months when more indoor lighting is used. Again the 'cooling degree days' math can be applied.

Spring and fall moderate temperature periods would be your baseline approximation for electricity and natural gas consumption. Little need for AC or heating during those times.

Tip: Do not just look at the dollar costs per time period. Work from the actual fuel/electricity usage as the pricing of these can vary from year to year and season to season.

if your utility has significant delivery and customer charges/fees those can be a significant portion of the bill total, and may not change in direct proportion to reduced consumption.