How can you tell if a speaker has impedence dips?

I had some 20's a few years back.
They arent real hard to drive.
At the time I had a Dynaco ST-70 @35 wpc that gave me good but not great SPL's.
The bass on these little guys is very tight and go deep enough for most pop,light jazz or light rock recordings.

I have read that some amps don't sound as good bridged, this could be another disadvantage if sound quality has to take a hit for more power.

I will explain that in simple principal of work of dynamic speaker that consists of magnet, voice-coil and diffusor.
The voice coil just by itself is an inductor coiled arround the magnet and hooked up to the diffusor.
Any deviation of a voltage between the voice coil ends will result a mechanical motions along the magnetic core as per Faraday's Law and swing the diffusor that will transfer the mechanical motions onto the air.
Every voice coil has a number of coils that defines its inductance with Inductive Reactance and Resistance accordingly. A Capacitive Reactance between the coils might be neglected for impedance dips explaination. The larger the number of coils the larger the Inductive Reactance and the larger the Resistance as well.
A Reactance is Variable part if Impedance that depend on freequency. Resistance is Constant part of impedance if considered at the same temperature.
Inductive Reactance is directly proportional to the freequency i.e. whenever freequency rises the reactance grows hence we'll have an impedance rise; and the other way arround whenever the freequency goes down the inductive reactance goes down as well down to zero for DC. In case with DC we only have a resistance of voice coil that defines Impedance.
Resistance is a parameter that does NOT depend on freequency and in some cases may be defined as the LOWEST speaker impedance that can be mesurable with simple ohmmeter accross the speaker's binding posts(pair of binding posts per speaker mmust be connected with jumper or wires in that mesurment case) and so will define the lowest "dip".

It probably won't effect anything really. The Paradigm's are of such a load that an amp will find them easy to drive. The only way to know impedance for sure is a test whether a review or manufacturer. If the amp runs hot, simply parallel the speakers. This will keep the impedance up. A bridged amp will like this load better anyway. When you bridge, your amp will see an 8 ohm load as a 4 ohm, a 4 ohm as a 2 ohm and so forth.
I do agree with the above poster about deterioration in sound quality when bridged.