All fires burn upward in an inverted conical shape. The point of origin is usually therefore the lowest place. Track from least damage to most damage. Theorize, calculate, and estimate time from ignition to flashover. You want to get some estimates of the volume of heat.
Odors are more important to trace than charred wiring.
Planned fires are designed to have the nearby presence of ventilation, combustible material, and sometimes accelerant. Melted copper, aluminum, and other metals usually means an accelerant was used. Soot that wipes off easily from glass or slightly charred studs behind the walls means a quick fire. What happens with a pipe bomb is that the explosion, being confined to a sealed container, produces a large volume of gas that actually causes the walls of the pipe to balloon and stretch until broken pieces of shrapnel fly out. Gas compression bombs of this nature have a strong shock wave effect, to the order of 7000 mph.