I always follow the following procedure for extra safety:
First I put on a medium sized condom.
Next I mix up a "warning paste" of deep heat, diced chillis and vinegar with a little curry powder to bind it together.
I layer the warning paste on top of the condom, being sure to get a full covering.
Finally I add a large sized condom on top.
The idea is, 2 condoms = extra safety and if there is ever a leak in either condom, you or she will know about it fast.
Be prepared. Do the responsible thing.