When fighting plasma races in fleet battles, do not forget to arm probes in information-gathering mode (one for each of the probe launchers in your fleet). You know, that little box on your SSDs that you usually forget about? No, you do not have to be crippled to use a probe in information mode. It is one point of power over two turns, and you have limited ammunition, but it is power and ammunition well spent.

A probe in information mode has a 360° launching arc, can be launched at a plasma torpedo up to six hexes away, and will automatically determine the target of that plasma torpedo. This six-hex distance is important, because at this distance (assuming your ships are moving speed sixteen or faster) you can still decide to turn away from the inbound plasma torpedoes to outrun them, or to reduce their warhead strengths with phasers.

In fleet battles, the plasma player is likely to have concentrated his torpedoes on a few of your ships. Once you know which ships are the targets, you can decide to have these few ships turn to outrun the plasma torpedoes. In the meantime, the remainder of your fleet presses on to obtain closer range to the enemy fleet. At closer range, fewer ships can likely cause more damage than your opponent's entire fleet at longer range. If your opponent has spread out his plasma torpedoes more evenly over the ships of your fleet, you will still be in a better position to decide whether to turn the entire fleet off, or to accept less damage on all of your ships and press on.

You still have to take into consideration the possibility of pseudo plasma torpedoes. Probes are not any more able to tell the difference between pseudo torpedoes and real torpedoes than any other system. However, you have to make that guess of real or pseudo regardless, and knowing the target(s) of the plasma torpedoes early is still a much better proposition for you.
Note also that you can use labs to identify the targets of plasma torpedoes, but labs are only reliable (better than 50% success) at ranges of two or less - and that is often too late to turn to avoid them without a High Energy Turn. Scout channels can also be used to identify the targets of plasma torpedoes out to Range 15. This function is highly useful for the same reasons given above, but uses valuable scout channels that could have been used on lending electronic warfare. Furthermore, scout channel identification only has a 50% success rate per attempt (four attempts per channel), whereas probes at range six have a 100% success rate every time.