Is method is available in errors package can be used to check the whether error is of type 'myException' or any custom exception which can return true for boolean check on Is(target error) method.

package main

import (
    "errors"
    "fmt"
)

func main() {
    var car = Car{}
    car.name = "Audi"

    var _, error = car.getMileage()

    if errors.Is(error, myException) {
        fmt.Printf("in custom exception %+v", error)
    } else {
        println("in generic exception")
    }
}

type Car struct {
    name string
}

var myException = errors.New("unable to compute mileage")

func (car Car) getMileage() (int, error) {
    return -1, myException
}

Output of this program would be

in custom exception unable to compute mileage