HOW TO CHECK FOR STRANGLES IN OVER-GROWN FIELD?

Hey:)! So, I'm gonna go and look at a horse on Friday and we've found a field to keep her in. It's quite over-grown but that's because there hasn't been any horses in there since December. The owner of the field said that if there was a horse in the field, it wouldn't be so over-grown. There has been strangles going about where I live in the countryside and I was wondering how you would check if there is any in the field? There aren't any other horses there so I can't exactly get a blood sample from them to see if they have it...