The chiyuv to spend money to check for shatnez, generally, is highly questionable. Brooks brothers suits, however, bc of the frequency of shatnez found in them, may be an exception.
Don’t ask shatnez checkers. They are technical experts, not rabbanim. It’s like asking a restaurant owner if they’re kosher when there is no hechsher. The financial incentive creates huge biases. Ask a qualified rabbi who is familiar with facts on the ground. And anyone who doesn’t answer you in terms of chiyuv (i.e. you are not mechuyav but a bal nefesh should spend some money vs. “why wouldn’t you, it doesn’t hurt?”) should not be answering the shaila.