I buy watermelon chunks in big clear plastic tubs. Watermelons are not a convenient food to store, so I find the cut up chunks quite convenient and delicious. You cn see watermelon passing freshness because it gets deeper pink and develops rounding on the ciut pieces. The "Sell by" date on a watermelon package usually has little to do with the peak freshness you want.
1I only buy the tub where the watermwelon chunks are deepest pink and no white showing
2All the cuts should be sharp and stand high
3There shouldnt be a lot of liquid in the bottom of the tub