They can't and it's never implied they can, it's shown in a couple situations.
1) Babidi and Dabra require the Kili Meter to sense SSJ Goku's power and are unaware of SSJ2 Goku's strength (Though it can be argued Goku was too far to be sensed, being in Yakon's planet and stuff).
2) Babidi and Dabra can't sense Boo's cloud forming above them, and Dabra thinks Boo has no power despite Gohan considering him a serious threat.
It's not the default for one to sense Ki, it's an ability mostly related to deities/good characters who have trained their spirites and such. Babidi and Dabra are shown to be sorceres/dark magic users and Babidi even possesses his own crystal ball, an object strongly related to divination. That even explains how he found out about the Saiyans: He most likely divinated who were the strongest between them, akin to how he somehow knew the Budokai would have people with enough strength to revive Boo, but didn't know who or how exactly.