If you had an unknown substance and you had to deterimine if it was an acid or a base, some properties you could look for if you thought it was an acid are if it tastes sour (although you shouldn't taste an unknown substance), if it reacts with metals, carbonates, or if it turns blue litmus paper red. If you were looking for the properties of a base, you could see if it tastes bitter (once again don't taste unknown substances), if it feels slippery, or if it turns red litmus paper blue.