Reference-wise I'd say it might be worth having a look in the front of "Coins of England" (i.e. the standard catalogue) by Spink, the last copy I bought did have some fairly good advice about how to spot fakes, but ultimately I think that experience is really the best guide. I know I have seen some on ebay that I was sure were fake just from looking at a picture of because you have a general idea of what actual silver looks like (it turns out I was right and someone, inadvertently I think, got some negative feedback). The other info you have on ebay of course is you know the sort of things that the person whose selling it usually sells and so you can to some extent rely on them; if they have over 1000 coin sales and 100% positive feedback I think you can afford to be fairly trusting.
That's not to say that I'd hold myself up as an expert, it takes many years really to have a really good hit rate on well made fakes and I think that on a pretty decent fake my chances of getting it for sure would be about 50% - not bad but not great. Of course on the really well made fake my chance of knowing definitively would be about 0%... I suppose this is the reason that they spent about 6 months analysing the Coenwolf I gold penny before it went on auction; it can be just that hard to tell even for experts with all the devices you could ask for to aid in the investigation. And if they struggled to say definitively you can bet that we would have no idea.
With regards to archaeological findings it's important to remember that there are really two types of "fakes". The new ones which come up on ebay, not worth anything really, don't tell us anything useful about any time other than our own (but you're probably a lot less interested in that anyway). The old ones which were made at the time are more interesting even though they are often of worse quality (I've got a Henry VIII coin made of lead which, obviously, has been rubbed pretty badly) but are far more historically interesting because it tell us more about the people who were around. A large quantity of old fakes would be a really interesting finding which would be pretty important because it may be evidence of large scale forging in the past which may tell us more about law and order in the time period.
"Hard to explain" may make the findings more interesting, remember though that coins could have been dropped and pushed a little way into the ground and so might seem like they are in slightly odd places (but obviously a Henry III penny shouldn't be under a roman floor). If you suspect that the metal dectorists might be actively lying to you then I guess it makes the problem more difficult. I think that if they are bringing in things like gold anglo-saxon pennies then you'd need to contact real experts anyway - but given how rare they are I might be suspicious at first. If they are bringing in slightly poor looking (i.e. "about fine") Henry III pennies then I'd say they have no reason to lie about it - they're not worth that much (I bought one for £5) so I think it'd probably be easier just to get a real job.
Still, if they are posing a lot of challenges for your analysis then you might still want to get them checked out. If you took them to a local coin dealer in person and explained the situation (i.e. that you're part of a historical society and aren't in it for the money) they might be willing to help you out for free just to give you a general overview of what's real and what's fake. Failing that appraisers will do it thoroughly but at a cost (Spink in london will tell you everything you could ever want to know about a coin and give it a professional grade - this will increase the value but comes at a cost).