The magic binary flags for good paired reads are 83,99,147 and 163. Those are properly paired, with one read forwrad, one read reverse.
If the software that you are using to make your sam files won't count mate pairs as properly paired, then you are looking for 81,97,145, and 161. Those will be reads where one is forward, and one is reverse, but where the software for some reason thinks that the pair isn't right, which could be a mate pair.
If mate pairs are being counted as properly paired, then you want pairs where the forward facing one points out, therefore, is ahead of the reverse one.
So in a normally paired run, all the read with binary flags of 147 have negative isizes. So a positive size should mean mate pair. Likewise, if the flag is 99, a positive isize means paired end, a negative isize means mate pair.
If the flag is 83, a positive isize means mate pair, negative means paired end.
If the flag is 163, a negative isize means mate pair, positive means paired end.