Each brokers have different “Liquidity Providers”.
The Liquidity Providers can be some mega banks or local banks. If a broker is not a complete market maker, these LPs should be introduced in the website.
Most of the brokers have several LPs and receive several price quotes, and they provide the best quotes to traders.
CFDs on cTrader
On the other hand, CFDs price needs to be quoted from a stock exchange.
So you can follow the markets and check out trading hours and history charts and etc.
In this way, you can also find out if a broker has put some markups on the spread.