There's no real short cut
I find the comparison websites useless, as I have a complicated history and several bikes on the one policy. They don't have the flexibility to cope. Maybe they are a shortcut for some, if the circumstances are straightforward.
You just have to resign yourself either to not knowing if you're paying more than you need to (almost definitely) or to spending time and getting frustrated ringing round a lot of different insurance brokers initially and then every time your renewal comes up. Depends how much you value saving money. Everyone's circumstances are different, so there's not much point in asking which broker is best, unless you want a list of candidates to ring round.
At my last (MCE) renewal, they swore blind they had given me their best quote, but I rang round and got a better one, which they eventually matched (going down about £50 to about £260, from what I remember). It's a pain in the arse. I felt like going elsewhere because they had messed me around, but changing companies also involves effort and I couldn't be bothered.