I will be quite honest here. I have had minimal training with regards DSEAR. If I see that a premises that has dangerous substances I look carefully at the controls they have in place, I expect to see the safety data sheets available for attending crews, and look at general storage, signage, specific procedures (Nominated fire teams, secondary evacuations etc), spill kits, nominated trained BA wearers in certain situations, capability/use of bunded areas, correct DSEAR zoning and the appropriate standard of electrics in the zones, information supplied to employees/contractors. If I was not confident that issues were addressed then I would be phoning for another officer who would be more experienced in the field, or even phoning the HSE to do a joint inspection. Common sense may go a long way, but it doesn't always stop you missing important details.
With regards COMAH sites, it has been raised as an issue within this brigade that now we are the enforcing authority we need competent officers to inspect them, and it seems to only suitable officers we have are the old HAZMAT trained operational officers who have had FP training also. (But these officers will eventually retire, leaving just us civvys with the appropriate FP experience but no HAZMAT training.) The problem with HAZMAT training is that the HAZMAT courses that are available are looking at the problem from the operational side only. The possible solution here is again, joint inspections with the HSE.