My Dog Just Ate Some Nasty Stray Cat Feces. What Should I Do?

He ate a little of it before I even knew what he was doing. There are a lot of stray cats around here and I’m concerned that he’ll get worms from it. He is on Heartgard, of course, but that doesn’t cover all types of worms. Should I have him fecal tested right away, or just look for symptoms? I think some types of intestinal worms don’t have symptoms though, right? I’m confused.