qual format determination

[Giles]

I typically map fastq files taken from the short read archive using bowtie on Galaxy. I'm now moving to the command line. For this, I'll need to know what type of Qual format my fastq files are in. I often get them from disparate sources. Does anyone know an easy way to figure out which format is present within a given file?
Keith.

[kmcarr]

If you haven't seen it there is a good graphic in the Wikipedia article about FASTQ which illustrates the ASCII character range for each variation. For most purposes though you need not be concerned with exactly which Illumina encoding was used. It is sufficient to know whether the Phred score offset is 33 or 64. A very simple test you can implement is, are there quality characters with ASCII < 59 but none > 74? if yes it is Phred+33. If the opposite holds true (characters with ASCII > 74 but none < 59) then it is Phred +64.