How do you know when a scrape is starting to get infected

A lot of swelling and redness around the area.

it could go all mouldy or swell up and turn red

The scrape could start smelling too.

The area would be red & warm/hot to the touch. Pain would probably be increased. There may be a yellowish discharge from the wound.

Try gently cleaning the area with warm water & soap. Do not scrub the area it can cause more skin damage. Once u have cleaned it thoroughly u could apply some Polysporin or some type of topical antibiotic cream. Then cover with a bandage. You should clean it daily & apply the cream daily as well.

If after 2-3 days the redness, heat & discharge do not go away or lessen u should see someone about it.

Hope it gets better soon.

it gets all red and puffy.
if you get a scrape you should clean it with like rubbing alcohol (even though that strings) or neosporin. :)