You have to get the name of the weapon that causes the death. If you are unsure what weapon the explosion if, use a printtochatall function to print out the weapon that caused the death.
I'm moving between places so I don't have my code base to reference from, but the line to retrieve the code would be...
Code:
new String:WeaponName[128];
GetEventString(DeathEvent,"weapon",WeaponName, 128);
PrintToChatAll("Method of death was %s",WeaponName);
Once you find the name of the weapon (event_death uses a different naming convention than standard!) that causes the death, just do a StrContains check for it in the death_event.
Edit: Poop. I completely didn't read your post. Sorry Dude. Although I find it strange there is no death event (I thought that was de-facto standard?). Are you sure you're not having issues with that instead?