I am a sniffer – happy to use milk up to failure of a personal test. But milk soes have two excellent warning devices built in – smell and taste.
Lifejackets are more difficult to operate the sniff test. The bladder test is simple and excellent practice annually.
The cylinders are also easy to ‘sniff’ as you can weigh them (certainly the mainstream suppliers quote empty and full weights).
Are they autos? If so the third critical element is the activator. Unfortunately sniffing these without letting them off is much more difficult. They can be visually inspected for corrosion / mechanical issues but not much more. Therefore for me these should just be replaced at their ‘use by’ date.
This PBO page provides a useful guide to diy checking. They recommend professional servicing but, other than being able to wave the certificate, I am not convinced this is necessary if you follow the advice in the article. Of course a different perspective would be that the additional £15-20 (maximum) for a professional service every two or three years is a small price to pay for peace of mind….
A final provocation. All of this ‘safety’ equipment is actually ‘recover after something has gone wrong’ equipment. If you have a sum of money to spend are you better to spend it improving the boat and its equipment (and your knowledge/experience) so that you can avoid needing to ever use safety equipment?