How can I tell if the machine that I'm using is heavily loaded?

You can use the mpstat command (on multi-processor SUN's) to see how busy the processors are:
/usr/bin/mpstat [ interval [ count ] ]
For example, using "mpstat 1 5" will return the activity of the processors 5 times, with 1 second between each sample. Generally, the first sample or two are not accurate. (The first sample shows the average since the system startup time.) An example is shown below for a 6-processor system. The last column shows the percentage of each cpu that is currently idle.
peta.ecn.purdue.edu 167: mpstat 1 5
CPU minf mjf xcal  intr ithr  csw icsw migr smtx  srw syscl  usr sys  wt idl
  0   17   0   83    13    0  218  175   18  140    0   297   40   3   2  55
  1   14   0  301    13    1  237  160   18  164    0    12   23   3   3  71
  4   13   0   55    13    0  194  166   18  153    0   379   33   2   3  62
  5   12   0  383    38   26  247  161   18  162    0     8   34   2   3  61
  8   13   0  379   219    9  183  130   19  156    0   323   28   2   3  67
 12   13   0   28    26   14  255  159   19  171    0   177   37   3   2  58
CPU minf mjf xcal  intr ithr  csw icsw migr smtx  srw syscl  usr sys  wt idl
  0    3   0   10     0    0   24    0    0    0    0    25    0   0   0 100
  1    0   0    0     2    2   39    0    0    1    0    24    0   0   0 100
  4    0   0    0     6    1   12    5    3    0    0     0   83   0   0  17
  5    0   0    0    10    8   16    2    5    0    0     1   17   0   0  83
  8    0   0   20   222   22   30    0    1    2    0    20    0   0   0 100
 12    0   0    0     7    0    7    7    0    0    0     0  100   0   0   0
CPU minf mjf xcal  intr ithr  csw icsw migr smtx  srw syscl  usr sys  wt idl
  0    0   0    0     5    0   11    5    1    0    0    15   83   0   0  17
  1    0   0    0     3    3   53    0    3    0    0    38    0   0   0 100
  4    0   0    0     2    0    3    2    1    0    0     3   17   0   0  83
  5    0   0    0     2    2    8    0    0    0    0     0    0   0   0 100
  8    0   0   20   219   19   34    0    0    1    0    20    0   0   0 100
 12    0   0    0     7    0    7    7    0    0    0     0  100   0   0   0
CPU minf mjf xcal  intr ithr  csw icsw migr smtx  srw syscl  usr sys  wt idl
  0    0   0    0     2    0    4    2    0    0    0     0   17   0   0  83
  1    0   0    0     2    2   58    0    0    0    0    44    0   0   0 100
  4    0   0    0     0    0    4    0    2    0    0    11    3   0   0  97
  5    0   0    0     9    4    6    5    1    0    0     0   83   0   0  17
  8    0   0   20   219   19   25    0    1    0    0    25    0   0   0 100
 12    0   0    0     7    0    7    7    0    0    0     0   97   0   0   3
CPU minf mjf xcal  intr ithr  csw icsw migr smtx  srw syscl  usr sys  wt idl
  0    0   0    0     5    0    6    5    1    0    0     0   83   0   0  17
  1    0   0 8590     3    3   40    0    0    1    0    31    0   5   0  95
  4    0   0    0     7    0    7    7    0    0    0     0  100   0   0   0
  5    0   0    0     6    4    8    2    3    0    0     0   17   0   0  83
  8    0   0   20   219   19   54    0    1    2    0    43    0   0   0 100
 12    0   0    0     0    0    0    0    0    0    0     0    0   0   0 100