Oldtime fiddle: powerful rhythmic shuffle style, not the smoother legato of bluegrass.
Oldtime banjo: percussive rapping/frailing or fairly unsyncopated picking, not the three-finger patterns of bluegrass.
Basically, if it sounds like Bonnie and Clyde driving away from a bank robbery, it's bluegrass. Otherwise, it's oldtime.
   In musical circles, you're only really allowed to like one or the other.(As in the 60's: Beatles or Stones, Dylan or Simon and Garfunkel).
As James Russell Lowell put it so eloquently on this subject:
Once to every man and nation
Comes the moment to decide
In the strife of truth with falsehood
For the good or evil side.