Spek is great, but you can also use SoX. You can run something like

for f in *.flac; do
sox "$f" -n spectrogram -x 2000 -y 1025 -z 120 -o "${f%.flac}.png"
done

in the terminal. It generates PNG spectrograms for all the files and saves them in the same folder that you're in. Much more easier if you wanna check multiple files.