You should directly ask the broker about their trading execution model. Most regulated & good brokers will be transparent about it, and also have page on their website for execution policy related terms.
Brokers that offer True ECN access accounts will mostly likely mention it openly. But this does not mean that an ECN broker is the best. Some brokers have hybrid model which can be a mix of STP & ECN, but still be a NDD (No dealing desk) forex broker, and are very transparent with good execution.
FxPro for example is a FSCA regulated NDD forex broker but they don’t have a direct ECN model. So, according to us what should matter most for beginner traders – is the broker is well regulated & does it have quick execution model at lowest possible fees.