Most commonly, tones are indicated by diacritics, which is how vowels are treated in abugidas, which is the case for Vietnamese (a true alphabet) and Thai (an abugida).
mˈoʊst kˈɑːmənli, tˈoʊnz ɑːɹ ˈɪndᵻkˌeɪɾᵻd baɪ dˌaɪəkɹˈɪɾɪks, wˌɪtʃ ɪz hˌaʊ vˈaʊəlz ɑːɹ tɹˈiːɾᵻd ɪn ɐbjˈuːdʒɪdəz, wˌɪtʃ ɪz ðə kˈeɪs fɔːɹ vˌiːɛtnəmˈiːz ɐ tɹˈuː ˈælfəbˌɛt ænd tˈaɪ ɐn ɐbjˈuːdʒɪdə.