Edit
「てへぺろ」 "tehepero" is a phrase coined by the seiyuu Hikasa Youko, a combination of the two onomatopoiea 「てへ」 "tehe", for a giggle, and 「ぺろ」 "pero", for sticking the tongue out. It is often rendered with a star 「てへぺろ☆」; the full emoticon is 「てへぺろ☆(・ω<)」, from her 2009 blog post
The archetypical facial expression and pose for tehepero incorporates a cheeky grin with tongue out, a wink, and either one arm up behind the head (the K-on! version, see post #784089) or a v hand sign raised to the face (Hikasa Youko version, see post #838257 or photo of her on Koyama Tsuyoshi's twitter)
Since winking and sticking the tongue out are both common facial expressions, an image should only be considered a reference to tehepero and tagged as such if the complete tehepero pose is emulated, or if a reference is made explicitly in text (i.e. 「てへぺろ」 in speech or sfx). If the character is tapping their head while winking and sticking their tongue out, use dojikko pose instead.