I am separated from my wife she lost custody of our four year old daughter because of drugs now that I am close to getting custody she has contacted another man and got him to petition for a paternity test to keep me from getting custody what should I do

Which state?

The laws vary from state to state.

Do you think a paternity test won't show you to be the father?

As GV said, laws vary, for example: in CA one has only a 2 year window to challenge paternity. In other states it can be challenged until the child turns 18