kwc, despite what some people would have you believe, designing, installing and maintaining fire alarm systems is not rocket science!
Obviously, you can only be considered as competent when you have enough knowledge and experience, and the correct tools to use your knowledge and experience.
Experience can only come with 'time on the job'.
Knowledge is readily available if you put the effort in to find it.
To get the required knowledge you should:
1) Obtain as much free literature as possible from equipment manufacturers. Much of this is now available on their websites. You should initially read those items that are general guides to equipment operation. Be prepared for some confusing information and errors in some of this literature but reading as many as possible from different sources will eventually guide you in the right direction. Subsequently read the equipment for specific equipment types to get an understanding of what it can do and where you might use it.
2) Search the internet for information on fire alarm systems. Also search for forums relating to fire alarms and read as many posts as possible. (you are obviously doing this already!) This will give you a flavour of the current terminology and the typical questions and answers to problems that people in the industry are experiencing.
3) Learn BS5839 by heart. This is probably the most important element of proving competence this days. As well as having a copy of the BS and reading it, also buy Colin Todd's guide to it (ISBN 0 86213 129 4). This will give you a bit of the background to how the standard has evolved and the thinking behind it. (I particularly enjoyed the section that explains why BS recommends installing a single sounder adjacent to the fire panel on one circuit and all other sounders in the premises on the other sounder circuit ).
4) To 'prove' your understanding of BS take the various BFPSA course modules. Unfortunately you can't just do the tests but you have to take their training as well, so even if you know what they are talking about it you have to sit through it just to answer the questions that will give you a certificate. However having the BFPSA module certificates is a good sign of competence
5) Finally many of the people who can supply your comapny with their fire alarm equipment have knowledge and experience that they will be willing to share with you for 'free' if you buy equipment from them. You are unlikly to find this knowledge at a general electrical wholesaler but you will obviously find it at equipment manufacturers (if you can buy direct) or from the specialist safety and security equipment wholesalers. never be afraid to ask a question.
Good luck in your quest for knowledge of fire aalrm systems and well done for recognising that you may have been lacking it!