water leakage suspected = mold concern

You won't know until it's too late,bleach kills everything. use it regularly until you are no longer concerned. Spray it, let it do its job

The smell would be the first thing you would notice.

If the tiles are in a shower the grout should be sealed after it cleaned. Or anywhere that water will be in contact regularly with the grout it should be sealed every 6-12 month to ensure it does not leak and break down the motar.

The best way to know is to inspect the area behind and around the tiles, especially if there has been water leakage in the past. Regrouting can help seal the surface, but if water has already gotten behind the tiles, there could still be moisture or damage underneath. I would recommend checking the surrounding walls and subfloor for signs of moisture, soft or damaged material, staining, or visible mold. A moisture meter can also help identify hidden moisture before opening anything up.

Regrouting doesnt always stop a leak. If water is getting behind the tiles there could be a waterproofing issue. I would check for loose tiles, soft wall areas or moisture first.