Plane Touched

Aasimar and tieflings are both uncommon, and to about the same extent. Tieflings appear more abundant due to their decreased ability to hide in plain sight. Aasimar have a tendency to look almost as extraplanar as tieflings in Cael, and the two can even be hard to distinguish to the untrained eye. They share a lot of traits in common—abnormally light hair, metallically colored sclerae, and strange colors on the skin. Both aasimar and tieflings are also very diverse in appearance: no two planetouched look alike. The key differences between the groups to look out for are as follows.

Aasimar
Aasimar tend to be patterned, not branded (whole-body colored). Many aasimar have decorative, almost runic symbols patterning their skin. However, this is often much more subtle than the branding sported by tieflings. Some aasimar may only have a symbol or two on their face that is easily passed off as war paint.

Aasimar's halos levitate supernaturally. Halos could take the form of a traditional golden circle but are more often something unorthodox such as small wings, a boney circlet, or jagged and angular metallic disk. Aasimar, beyond their symbolic markings, can hide their extraplanar origin. Their wings and halos appear in a spectral fashion only when the aasimar so chooses.

Aasimar have the ability to hide. Although the same neutral aligned extraplanar deity might be able to make a tiefling and an aasimar just the same, the aasimar is given their form as a blessing, and in being blessed they have a choice.

Tieflings
Tieflings tend to have startlingly irregular skintones, such as red, pink, and purple. These brightly colored tieflings are also known as "branded" tieflings, as it is believed their bright and showy appearance indicates a more recent fiendish involvement than can be attributed to more subdued tieflings. While not all tieflings are brightly colored, one would never find an aasimar appearing solid red.

Tiefling head appendages are bone and grow directly from the skull. Tieflings also tend to have barbs or spikes where Aasimar have feathers. Tiefling horns (and wings, if applicable) are flesh and blood and mark the tiefling always.

Tieflings are given their form not as a blessing, not as a curse, but as a reminder.

Further
There are some exceptions to these rules that make it easier to confuse the two groups. Young aasimar, or those with unstable power, may not have total control over their angelic forms, and reveal themselves by mistake--or not be able to conceal themselves at all.

And some (non-branded) tieflings may have patterns on their skin like aasimar do, as a result of descent from a particularly flashy "branded" tiefling. Generally, the safest way to tell the difference between a devil and an angel is to look for bright, solid colors and boney horns. However, if a tiefling is not one solid bright color and has smaller or more unorthodox horns, they could pass for an aasimar. Similarly, an aasimar could be taken for a tiefling depending on the visibility and form of the halo.