It's one thing being with your friends from the beginning of a project, but getting pitched an idea and then offered to become a co-founder can be shaky.
Is an even split still expected?
If you're not there from the beginning I would not expect an equal split. The longer the startup has been around, the less equity you can expect to get.
I like to look on the brightside of things. Being a skeptic can be harmful. If your gut is telling you that something isn't right, than be careful.
Trust your instincts.
If they've been busy for a while and they've passed through one or more cusps would mean that your risks are reduced, on the other hand if this start up is two weeks old then I'd lean towards an even split, depending on what you bring and whether or not the others have 'an even split'.
If you're being approached then that would be based on reputation and talent, have a long hard look at the business case and if you believe in it then you can weigh your options.
Make sure if you have a s.o. that they're on board, start up life can be pretty stressful.
Best of luck.
Bill Joy joined Sun after the founding. One of my partners joined us later, too, They are absolutely cofounders.
That said, the biggest red flag? Dishonesty. If you smell a whiff of it, I would advise serious caution.
You're right though, I probably should have said 'prewage' :-p
If all the have done is pretty std business startup crap a monkey with a phonebook and checkbook could do, that does not really count as risky work