Speaking from my own experience, I know right away that the LoRs from my supervisor and my dentist would be stellar even though I never read them, it's just something you could naturally tell when you asked the recommenders. The LoRs from my science profs would be decent since I got A's from their classes, but I don't think they would be called "strong" since they don't know me super well. That's how you guess the quality of your letters. Generally people don't give out negative letters, otherwise they would have refused in the beginning. So don't worry too much, yours are probably fine.