As with any of these things I feel it's good to know how existing members feel.

Is there a testimonials page?
Are you able to contact others to ask them about their experience?
Do they have a forum where members can chat to one another and learn from one another?

Most of the VA networks do but these other sites tend to be more farming sites for picking up paying customers (prospective VAs) but give little back for their fees.

There are genuine VA networks where membership fees are required but they always have a list of what you get for your fees, and it isn't being introduced to jobs that could be old or one-offs. And there are always contact details for those who are running the networks.

So it's good to do your homework and check up on these - the great thing about coming to a forum like this one!