This is something that I definitely fear as well. Although the chances of infection is pretty low if you make sure that your dick is clean before you stick in it your doggy girl and abstain from sex 2-4 weeks after a heat cycle. Usually I'll hop in the shower before I make love to my doggy girl. Usually with infections you'll see signs beforehand, such as discomfort, lethargy, as well as them not wanting to be touched down there. My biggest worry is if I make love to my girl and she then trips down the stairs on her after sex walk and breaks her hip. Something that's unexpected, and that I cannot prepare for beforehand. Although my first worry will be getting her to the vet ASAP.
They can, but from what I've heard it's usually unlikely. In theory they could ship a sample of whatever leaks out of her vagina to a lab to be tested, but most of the time they won't bother with doing so. It definitely helps if you take a clean paper towel and wipe the inside of her vulva from top to bottom. As usually your sperm will collect towards the bottom of her vulva near the opening of her urethra before it leaks out. That way you can try to collect any loose semen.
Human cum and dog cum will typically have a different consistency. Human cum is thicker and whiter. While dog cum is typically thinner and watery. If your semen leaks out, any trained vet will probably know the difference. But there is a possibility they might teat human semen as simple "unknown vaginal leakage." They'll probably run a blood test, and if nothing seems amiss, they might just tell you about the leakage, and to keep an eye on it. But you can never know for certain. I heard this from a vet tech.
There's always a risk with being sexually involved with your female doggy girl. I take a slight risk every time I make love to my girl. But usually if you keep yourself clean, make sure you can fit inside by fingering her first, and make sure you keep an eye on your girl for any changes. The risk is pretty minimal. But the risk is always there, sometimes life will throw a few curveballs at you. But the best thing about ZooVille is that there is a LOT of information to be read in the "Health and Wellbeing" and "How To's and Educational" sections of the forum.