distinguishing a power transformer and a reactor

A transformer will have at least two windings, therefore at least two pairs of terminals, and will have either a turns ratio or an input voltage and output voltage marked on it.

Visually, externally, it could be difficult. A Delta-Delta 3 phase transfo [and many configurations without "neutral" terminals] will have 3 terminals input and 3 terminals output So will a 3 phase reactor. And both can be in oil tanks. The difference is that the transfo will have isolation, input to output, a reactor will have connection input to output for each phase. The fundamental difference, visible without the tank, is that a 50/60 Hz power transfo has an iron core for its windings - a reactor is without iron core so its reactance is sustained even at the overcurrents it is designed to carry during a system short circuit. Limitation of short circuit current is the usual purpose of a power system reactor.