(It doesn't really matter when writing informally - especially on the telephone. Many written communications can appear to be exceptionally lazy... eg, slt mlm for "good night / selamat melam".
Learning the text-speak of the language is a whole other language, sometimes :p
You can specify it by adding "sedang" ("right now") before the verb. In a similar way, you can send it to the past by preceding the verb with "sudah" ("already"), or into the future by using words like "akan" ("will") or "nanti" ("later"). In general, the "time words" will tell you the tense. But the verb stays the same.