A brood frame? Or the whole broodnest? I'll choose the whole broodnest ok?
I look for pollen and open nectar stores above and to the the sides of the broodnest, and make sure the larva is not drying (I get some nasty dearths here in late summer).
I look for eggs, which tells me the queen was there or not within the last three days.
I look at the pattern, how solid it is, both for larva and for sealed brood. Spotty larva and sealed could indicate excessive same sex aleels (spelling?). Solid larva but spotty sealed could indicate good hygienics with varroa mites, or could indicate excessive hygienics (such as with VSH x VSH queens).
I look for varroa mites!
I look to see if the queen has enough room to lay eggs and/or look to see if the queen is laying enough according to the amount of nurse bees present in the hive.
I've been keeping a few hives for the past 20-25 years, and no longer treatment free as I got tired of the losses.