I'm buying an electric convection oven how can I tell if I have a single or three phase power?

If you are in the US and this is a home residence you will have single phase service.

Even very few commercial locations in the US have 3 phase, if you have it, you should know it, since you would have paid a large sum to have it installed.

Look in your breaker box at top where main power switch is located. Three-phase switches have three legs and single phase units have two legs. So, it's either a double or triple breaker.