Could we see evidence of a past vacuum metastability event? As far I know, if such an event would happen, we couldn't see it coming:

A hypothetical vacuum metastability event would be theoretically
  possible if our universe were part of a metastable (false) vacuum in
  the first place, an issue that was highly theoretical and far from
  resolved in 1982. A false vacuum is one that appears stable, and is
  stable within certain limits and conditions, but is capable of being
  disrupted and entering a different state which is more stable. If this
  were the case, a bubble of lower-energy vacuum could come to exist by
  chance or otherwise in our universe, and catalyze the conversion of
  our universe to a lower energy state in a volume expanding at nearly
  the speed of light, destroying all of the observable universe without
  forewarning.

What about one or more that might have happened in the past, and engulfed the visible universe?
 A: There could indeed be tell-tale signs of a phase transition in the early Universe, particularly if it were a first-order transition. 
If it were first-order, it would involve bubble nucleation, much like boiling water though during cooling, and latent heat, much like phase transitions for water. The bubbles are regions of space in which a phase-transition to the true vacuum occurred.
The latent heat could, ultimately, have been released in collisions between bubbles, resulting striking gravitational wave signatures that could be observable at gravitational wave interferometers, such a LIGO and LISA.  See e.g., astro-ph/9310044 for an early work on this topic. At present, only the LIGO experiment is in operation, and it detected only gravitational waves from black hole mergers.
I don't know of any way to detect second-order phase-transitions, though, as they don't result in latent heat, bubble nucleation or gravitational waves.
A: Another way to test for false/true vacua is via collider experiments. High enough energy collisions could create a bubble of the other vacuum. A (relatively) low energy collision will only perturb your field slightly around the local minima of the potential in which that field resides. On the other hand, a high enough energy collision will no longer be a perturbation around the local minima, but instead have your field cover the entire potential. If the field settles into a different minima, you will have that field temporarily residing in a different vacuum state (e.g. you just created a bubble of another vacuum state). When this bubble decays, the decay products of this bubble could give information on the global properties of the potential. 
