ECN vs STP labels: what actually changes for a scalper

[FXS London Scalper]

Every second broker website now says "true ECN". A few years ago it was STP. Before that it was "no dealing desk". The labels change faster than the execution does.

What I check instead of the label, on a small live account, before I trust anyone with real size:

1. Commission plus raw spread versus a standard account on the same server. If the all-in difference is under 0.2 pips, the "ECN" pricing is probably just a repackaged markup.
2. Fill behaviour on limits. A venue-style model should give you partial fills now and then. If every 2-lot limit fills in one piece at the exact price, you are almost certainly trading against the broker's own book. That is not automatically bad, but it is not ECN.
3. Slippage symmetry. Log 100 market orders. On a genuine pass-through model you will see some positive slippage too, not only negative.
4. Stop and freeze levels. Real aggregation rarely needs a 2-pip freeze level on EURUSD.

For a scalper the practical difference is cost and fill honesty, not ideology. I have had decent fills from brokers that openly run a hybrid book and terrible ones from brokers that call themselves ECN on every banner.

Does anyone keep a simple test like this when switching? I'm curious which checks you found actually predict the long-run cost.