The server's detection methods are not publicly documented for obvious reasons, but historical evidence suggests a combination of:
Focus on maps with high monster density rather than just rushing the boss. project diablo 2 maphack
Join the PD2 Discord. Ask veteran players for their “map strategy guides.” Use the legal loot filter and learn tile sets. You’ll discover that what felt like a need for a hack was just a need for knowledge. The server's detection methods are not publicly documented