Slippage does not always mean a bad broker, but frequent one-sided slippage, off-news rejections and missing records are warnings.
Around news, market open, close, low liquidity and price gaps, market orders may execute at the next available price.
Slippage only against the client, stop losses widened often, profitable orders rejected, platform cannot provide execution records, or support replies only with templates.
Save order ID, quote screenshots, execution time, server time, instrument, size, stop settings and exported trade history.
Compare multiple quote sources at the same time. Do not conclude from one trade only. Repeated samples matter more.