[georgyo]

The 23andme kit requires a substantial amount of spit, so it would be hard to trick someone into giving you their DNA.

In terms of doing DNA testing behind someone's back, paternity testing is DNA testing, and only requires a sample of the DNA. You can buy those kits at walmart already. Search for Identigene DNA Paternity Test Kit.

[tobylane]

This kit lets you collect material by yourself. Some other kits/companies require a doctor/nurse signature that they did take material from someone who matched a photo id.