How do I know if it is a mp3 file?

I have a Sansa Shaker, I have been trying to put songs on it for a week now.  I have downloaded only songs that say mp3.files, but when it is put on the shaker it says that it is a wma.file.  My computer also says it is converting songs to mp3 but it apparently is not.  So how can I tell if it really is a mp3.file?  Why is it not really being converted?  and why did Sansa make a kids mp3 player so hard to use.  I hope this is resolved or Sansa will be getting their shaker back.

If you right click on a song and select Properties the window that pops up will show you the file type.

What on your computer are you using to convert things to MP3 format?

I am using windows media player, first it says converting then, sync, then copying.  I have right clicked all the files to see if they are mp3, they all say they are but when they are on the memory card it says wma instead.  This is extemly frustrating because i have heard that other mp3 players are much easier to use, like they will except wma files, which would of course completly solve my problem.  I read on this web site that some wma files are over written with mp3 and that is what my true problem is.  That is why I want to know how to tell if it is really a mp3 file.  They ALL say mp3 file until it is on the memory card then it says wma file, if there is no other way than downloading all those songs to find out once I get it on my memory card that it is a wma file then having this thing is pointless and I don’t want it.  I am not going to keep downloading 100 false mp3 files, then deleting them. I will just buy a mp3 player that accepts wma files and Sansa can have this retarded thing back.