Really good question. Obviously we ( as DSL) have to train through our LSGB and then train up our staff, which can have some issues ( https://designatedsafeguardinglead.com/?s=inset) but its good to hear specialist in certain fields, which can be really tricky. Word of mouth, experience and some have even suggested I watch them deliver inset at other schools. Quality is key. Safeguarding is hard enough to deliver. if the speaker isn’t on topic, have the correct info or engaging then you can loose staff support very quickly.