I don't really have time to go and look up specific examples on Epitaph for you but i'll try and help you out.
Some common modulations that you'll find are:
Going to the subdominant. (changing key to the fourth, so if you're a guitarist it's like going from say the 3rd fret on the 6th string , to the 3rd fret on the 5th string. So if you were in E standard, thats a key change from G to C.
Another common change is going to the dominant, which is the fifth, so on guitar its like changing key to the top note of a power chord, say go from the 5th fret on the 5th string to the 7th fret on the 4th string, you can also get the same by going from the 5th fret, 5th string, to the 5th fret on the fifth string, so again in E standard, this is going from D to A.
Other common key changes are to move up or down by a semitone/tone. So go from D to E, or E to F, or F to E. etc. So on guitar thats just moving up or down one or two frets on one string.
You'll find that in death metal you're mostly in minor keys. When you are lucky enough to be in a key, necrophagist are usually quite tonal so they tend to stick in key and not bring in too much chromatic stuff.
Theres a really good example of modulation and stuff at the end of the title track of Epitaph, where it goes to the classical part. It's a modulation to the subdominant i think...thats just by ear i'd have to look at some music to tell you properly. But that would be a good place to look as its dead obvious whats happening.
I hope you find this helpful, but god damn, death metal for music classes is always a nightmare! I love metal, but gahh i'd never be able to cope with trying to get my music teachers to listen to it properly to work with it.
Good luck! You can find loads of stuff online which might help too.