Usually when a broker is STP, ECN, NDD or DMA - they’ll have it stated on their website.
I and @TradeViper have listed some traits that should help in finding one.
There are also other threads on the topic.
How to check for sure?
Perhaps you can ask for a post-trade execution report. However, that’s only an option if you’ve got a live account.
Make sure to contact their support and ask if they will provide such a report upon request BEFORE opening an account.