How to check if the pin headers are soldered well?
I don't see any obviously bad solder joints on those pictures.  However, these pictures aren't great for inspecting the solder joints.  To get pictures of small things, you need to use a macro lens, good lighting, and proper exposure.

The main thing to look for is that the solder flowed.  That is best seen in the top picture.  It seems the solder wetted and flowed on both the pads and the pins, although too much solder is clearly evident on many of the joints.  Too much solder makes it hard to tell the difference between a blob that wetted and flowed over the pad, and one that is just sitting on top of the pad with a layer of flux or oxidized solder in between.

The proper procedure for hand soldering such pins is to put the iron in contact with both the pin and the pad.  The feed a small amount of solder into a crack between the iron and the pin.  The purpose of this little bit of solder is not to make the connection, but to provide good thermal conductivity between the soldering iron and what it's trying to solder.

After a second or two, the pin and pad should be hot enough to melt solder on their own.  Feed a bit more solder.  This should now melt easily and flow all over the pin and the pad.  Remove the iron as soon as the solder has flowed.  While the joint is cooling, make sure nothing is wiggling the two parts of the joint relative to each other.

Added

On closer look at the third and fourth pictures, the solder joint for C15 is suspect.  At the least, the solder doesn't seem to have flowed over the whole pad.