5 Answers

99% of time that scar will be from an Ovihysterectomy, but to rule out another abdominal surgery, a simple ultrasound can be performed to verify that she is spayed. 

In the female, the abdomen is clipped free of hair, an incision is made along the midline of the abdomen, and the ovaries and uterus are completely removed;

Source >http://www.gladesvillevet.com.au/pages/brochures/information-leaflets/information-leaflet.php?idArticle=9883

I've seen that scar and it seems to me that vets have different styles when it comes to spaying and neutering!

My sister's dog was cut up and down. A dauchsuan.(sp?) don't know if breed makes a matter, I guess it's up to the doc.................But God bless   you for thinking about another dog..........

I don't know the first thing about it, but there should be a record of the spay with the vet who performed the surgery.  Ask for the vet's name, or a certificate of spaying.  They do give those where I live.