Open the door of the electrical panel and there should be a main breaker at the top of the breakers. On the breaker it will say the amperage. This applies to fuses as well. If it’s not marked or you don’t have a main breaker which could be the case with an old panel you’ll need to take the cover off of the panel and check the wire size. Generally it will be a number 3 wire If it’s copper or number 1 if aluminum. There are variations in wire size based on the temperature rating of the wire. You could have fuses as well. I would recommend if you don’t know what your looking at and you need to open the panel hire a home inspector, electrician or contractor to check for you it is dangerous.