Good advice below. Particularly the points about equity having absolutely nothing to do with a home's value, and even if you found a spread between purchase price and current value, the owner may have refinanced (reducing the actual equity).
Look: Any home bought after 2005 is likely to have no measurable equity. Homes bought between 1995 and 2005 probably will have some equity so long as the owner hasn't refinanced or didn't take out a big HELOC.Homes bought before 1995 should have substantial equity--so long as there's been no refinancing or there isn't a big HELOC. But that's just a real rough guideline.
Hey: You make an offer. If it's too low, the seller says "no" or counters. It's not rocket science.