The short answer is that you will not know it's safe until after your vacation is taken. Until then, there is a risk that the
DVC member will take your money and cancel the reservation, leaving you with nothing.
That said, I only recall one instance of that ever happening to a renter. Every other reported "problem transaction" involved a renter who did not follow through on his/her agreement and left the owner holding the bag. There is trust needed on both sides of a rental transaction.
To minimize you risk, most here will recommend that you have a written agreement - mostly to make sure both sides clearly understand the terms, especially the cancellation policy. Most rental transactions are non-refundable.
After a reservation is made, the owner should send you a written confirmation from Disney showing your name on the reservation. If you like, you can call periodically to make sure the reservaiton is still there.
It is a good idea to talk voice to voice with the owner and to exchange contact information. If you meet the owner here on the DIS, read some of his/her posts and see if they reflect the kind of person you would want to do business with.
Be sure you read all the sticky threads at the top of the Rent/Trade Forum before you agree to anything.
If after all that, you are still not comfortable, you should not rent. It's not for everyone. Those who cannot accept the risk, should book directly through Disney.
Best wishes-