Check out the finances over the last three to five years and you've got a pretty good idea on how much the "reserve" in the wet rate (with this I mean you take the wet rate, deduct direct expenses such as fuel, so that you are left with an amount of money that goes into a reserve fund, regardless of whether this is called "engine fund" or something else) is enough to cover the unexpected expenses, or the expected but unpredictable expenses (such as an engine overhaul).
Very easy to do this. Look at the financial statements and point your finger at any income of the group that's not the monthly fee or the wet rate. Ask questions, see if you're satisfied with the answers.
I'm with foxmouth on how to value the engine fund. Suppose you have two groups who fly completely identical aircraft - their serial numbers are only one-off. Each aircraft is in the same condition with the same equiment, number of hours flown and each engine is exactly half-time. Both aircraft are officially and accurately 1valued (if such a thing would exist) at, say, 20.000 euro and this value takes into consideration the current time on the engine. Both aircraft are operated by a group of four, but the groups are managed differently, leading to the situation where group A has an "engine fund" which contains, at this moment, half the expected amount of money for an engine overhaul. For arguments sake, assume the engine fund contains 2.500 euro. The other group is managed differently, and has no engine fund at all - it relies on the members to cough up the money as and when required.
Both groups have a 1/4 share for sale. Which share shall I buy?
If I buy into group A, I get a 1/4 share of the aircraft, valued at 20.000 euro, and I get a 1/4 share in whatever the other assets are of the group, meaning 1/4 share in the engine fund. So a fair value for this share would be 5625 euro (1/4 of the airframe + 1/4 of the engine fund).
If I buy into group B, I only get a 1/4 share of the aircraft, but no share in any reserves because there aren't any. So a fair value for this share would be 5000 euro.
If the shares for A and for B are indeed priced as calculated, then a second consideration would come into play. And that's whether the group is well managed, has its finances in order and has an idea on the future costs. With group A, at least somebody has done some homework and came up with a plan. Whether the assumptions in that plan come true is another matter, but at least someone tried to forecast the future costs and build reserves for that. With group B, although the share price is lower and the hourly rate is lower, I'm also buying into a big liability, because I have no idea whatsoever on the additional costs I'm going to have to pay for in the near future.
I do agree with bose-x though, in that the engine fund and any other reserves are ring fenced. They cannot be used by the members as a checking account for paying their daily bills and such. In fact, if done properly, the administrator of the group should produce a balance sheet regularly (at least once a year) with separate entries for "engine fund", "paint job", "unexpected repairs" and a number of other reserves split out, even though the money might be collected into one big checking/savings account. But that's just good bookkeeping.
Obviously not all costs are predictable, as IO540 has said. There's a thread somewhere here on new aviation sayings and quite a few are applicable. One I like is "an aircraft is a hole in the sky that you throw money in." The older and more unique the airframe is, the more the costs become unpredictable, but they're always higher than expected.