If you're able to contact the previous owner, they would probably be the best person to tell you when and if they need removed.
There are stitches that dissolve over time, so they might not need to come out and that's why the previous owner forgot to mention it. But, I don't know of any way to tell whether or not they're the kind that dissolves.
If you have the vet records that say when she was spayed, the stitches will need to be taken out a week after that date. If they're left in too long, you could start to see the body react negatively since the wound is healed and now there's a foreign substance in the skin.
I personally would take her to the vet to have them take out the stitches, just because I'm not sure I could take them out in a way that wouldn't be painful. I've also never had a vet charge me for taking stitches out. Especially if you can't be sure what time that the stitches were put in, I'd be nervous about taking the stitches out too early.