Sweetie, you can't tell. I wish we had an easy answer. I'm like your provider. I'm a germaphobe, use bleach in my dishwater, bleach my floors, have the kids help me wash toys when we see them mouthing them, and we feed tons of veggies, fruit, orange juice etc.
I only care for 4 all day and 4 all night with a few extras that share spots by alternating days. And yet, we get this stuff coming through at random and keeping kids home or sending them home NEVER works. Just today I had 3 children come down sick all at the same time and they were fine all day and sick by tonight. We don't know where they got it because the other kids seemed fine.
Hang in there. It gets better when they get older.