Your beer will be fine, even if some hops fell overboard. You may lose some bitterness, but it will still be fine.
I checked by floating my hydrometer in the bucket before, but that was before I realized that I had to shake the holy hell out of it to get the wort to mix with my top-off water. Since doing that, the wort in the bucket is WAY too frothy to do that. I think you are best off drawing a sample out to test since it will either (1) take forever for that froth to die back enough to get a reading or (2) mean that you are testing unmixed wort+top-off water and will get a bad reading. My last few batches, I used a sanitized turkey-baster to suck out enough wort to test in the hydrometer jar. You can get the actual mixed wort this way, and get a good measurement.