You can hire an electrician to come in and and do a power audit, or you can do it yourself. There are a ton of "how to do your own power audit" help sites on the net.
Once you have a rough idea of your energy usage, subtract this from the number on your main breaker.
Keep in mind that energy can be "shuffled around". As an example if you have a hot tub and your usage is approaching the limit, you can time the hot tub opposite your vehicle charging.
Another point to make here, it is often cheaper to upgrade appliances to use less power than to upgrade your mains. Replacing an old 240v water heater of clothes dryer with an efficient heat-pump model can get you more than enough "extra" headroom for a L2 EVSE.
Most good EVSEs can also be adjusted to match how much you wish to draw from the mains. If you have the power, a good 10kw + EVSE is a great luxury. If you are power-tight, many can be dialed down to as far a 3kw; which is often enough for a residential L2.