Really no way to tell unless the engine is sitting in its original application. Even then, it could still be difficult without cracking it open.
I wouldn't worry about trying to find a four bolt block anyway. The majority of the small blocks out there are 2 bolt and they will work just fine in most performance applications.
1969 Camaro - Intense Blue Pearlcoat/white (Baldwin-Motion paint scheme), stock 6.2 liter L92 with VVT, LS3 intake, Doug's Headers; Tremec TKO, 3.73 12-bolt posi.