For culture, looking for urine infection, traditionally midstream urine, or at least clean catch.  Hard if not toilet trained.

New urine collection method –

• Feed first!
• Then hold the baby under their armpits with their legs
  dangling. Another person then starts bladder stimulation – gentle
  tapping in the suprapubic area at a frequency of 100 per minute for
  30 s.
• The third step is stimulation of the lumbar paravertebral zone in
  the lower back with a light circular massage for 30 s.
  Repeat as necessary. 86% successful within 5 minutes, mean
  57secs!

[ [Madrid Infanta Sofia hospital, as reported in Arch Dis Child
2013;98:27-29 doi:10.1136/archdischild-2012-301872]]