Hi there.

I have an album in WAV Format here, but I'm not sure whether it's simply decoded MP3 or truly directly from a CD.

Is there any tool to check this?

Tau Analyzer could give you some clues.

great, thank you!

i've made an audio cd image mounted it with daemon tools and tau analyzer says it's CDDA

While it's more accurate than other software I've seen, you can't neccesarily take that as read.

don't mp3 decoders (at least lame) have low and highpass? so if you've got something above 18khz (could be wrong about the number, but pretty sure) it's probably not from mp3.

For have sure that's wavs aren't from an mp3 or another lossy format the frequencies will have to go at the limit 22.5 khz if they for any reason they stay below from this are from an lossy format.
NOTE:--alt-preset insane have lowpass at 20 khz and with lame is possible disable it,but sometimes the bitrate is not enough and the frequencies can't go on at the top with it disabled(22.5 khz).
And about the highpass the lossy formats "cut" some of the lowest frequencies(0 at 2 khz) but not at all just a bit.If you look at spectral view in a program like Sounforge the lowest frequencies still there but not at all.

Although based on the same code as Tau Analyzer (by True-Audio Software,) Audiochecker should be mentioned. This checks ape, flac, shn, wav, & lpac files as opposed to CD's. It has reliably detected flac's encoded from an mp3 source for me. It takes a while and is a CPU hog, and probably just uses techniques already mentioned in this thread.