There are a couple of ways.
Are you comfortable when it's really hot or cold outside? If so, then it's sufficient.
Do a Manual J heat load calculation. You can do a crude approximation of this yourself using http://www.loadcalc.net/
Use "rules of thumb." These rules depend heavily on your area and become less accurate the better or worse your house's thermal envelope is.
You have a roughly 3,000 square foot house, assuming the basement isn't insulated/sealed off from the first floor. And you have a 30,000 BTU heat pump. That means the primary HVAC system can provide 10 BTUs per square foot. If you house is average to below-average construction in terms of insulation and air-tightness, this would most likely be insufficient in Minnesota, Maine, or Montana. But it would be probably be fine in California, Florida, or Georgia. In the middle third of the country, it might be okay, might not be. Might depend on how cold it got.
The fact that you have a (relatively) low BTU central ducted heat pump coupled with an electric furnace tells me that you live in a cold climate, and that the people who installed this system suspected that the heat pump would occasionally not keep up during cold snaps, and added the electric furnace for back-up. It woulda been a heck of a lot cheaper to simply calculate the load correctly and install a right-sized heat pump in the first place...
Another thing to consider is that a lot of older heat pumps don't work very well below certain temperatures. So if it gets to be -10 outside, the heat pump might be incapable of producing any heat. Newer better Japanese heat pumps can manage this without electric, gas, or wood backup heat, but most older American ones can't. So if it gets really cold, you might be forced to rely on the electric furnace, which will cost you big bucks to run.
I would highly suggest you improve your house's insulation and air sealing. After all of that, you might discover that your heat pump works just fine to heat and cool the house.