How do you determine if a variable is a slice or array?

Have a look at the reflect package.
Here is a working sample for you to play with.

package main

import "fmt"
import "reflect"

func main() {
    m := make(map[string]interface{})
    m["a"] = []string{"a", "b", "c"}
    m["b"] = [4]int{1, 2, 3, 4}

    test(m)
}

func test(m map[string]interface{}) {
    for k, v := range m {
        rt := reflect.TypeOf(v)
        switch rt.Kind() {
        case reflect.Slice:
            fmt.Println(k, "is a slice with element type", rt.Elem())
        case reflect.Array:
            fmt.Println(k, "is an array with element type", rt.Elem())
        default:
            fmt.Println(k, "is something else entirely")
        }
    }
}