Actually, that might NOT be necessary.
Here's an easy way to check if it's looped.
Firstly, if it's a WAV file, rename the extension (not the filename itself) to lwav (that's Lwav, not iwav). E.g. if you have 'song.wav', rename it to 'song.lwav', but make sure you rename the extension only. Otherwise, you'll end up with 'song.lwav.wav'
Then, open it up with foobar2000, right click the song in the list, and click properties, and then click the properties tag again.
Go down to the bottom of the window. If you see something like the following, it already has loop points:
<LOOP_END>
<LOOP_START>
<STREAM_TOTAL_SAMPLES>
If, however, you only see <STREAM_TOTAL_SAMPLES> it doesn't have a loop point.
Of course, if this is too complicated, just rename the extension to .lwav, and use a converter which convert between loopable formats, such as ADX, AST, AAX, and even BRSTM itself.
Just don't use BCSM-GUI. There are many reasons:
1. It's extremely out of date (the MOST recent version is from July 2009 as far as I know)
2. It does not adjust loop points to a multiple of 14336
3. It ruins the quality (by making it sound TOO centered, or too mono, or less stereo, whatever you want to call it).
The easiest converter and the one that I use is this:
https://mega.co.nz/#!0JpzRKob!ZnTIMSGreYyxyA4A0feHkBz4iG50ck4ap42QGJWG78U
Included, are 3 batch files.
brstmloopfix.bat, dsp2brstm.bat, and lwav2brstm.bat
dsp2brstm.bat and lwav2brstm.bat do exactly what they say. They convert *.dsp files to *.brstm, and *.lwav to *.brstm respectively. And it automatically adjust loop points to be a multiple of 14336. Therefore, when finding loop points, you don't have to worry about that part. As long as the loop points you found are smooth, it'll work.
brstmloopfix.bat technically converts *.brstm to *.brstm. This may not make sense, at first, but essentially, it fixes loop points that aren't a 14336 multiple. The reason these are included, are because I just included it when I sent it to someone else. The lwav2brstm.bat is useful, if you wish to convert multiple files at once, however, you MUST have the *.lwav files in exactly that same folder as the batch files.
Essentially after downloading the ZIP file, extract it, then rename those WAV files to have a LWAV extension, drag those into the same folder that you extracted the ZIP files to, and then click lwav2brstm.bat.