>Is there an API call that determines if any certain file type is being in used?

You can use FOPEN.

lcFileName = GETFILE()
lnFH = FOPEN(lcFileName, 12)
IF lnFH > 0
  =FCLOSE(lnFH)
  * Ok to work with the file
ELSE
  * The file is open somewhere exclusively
ENDIF

Check also Who opened what files on the network? (modified to use only VFP code) FAQ #7896