@FuzzeWuzze: That might no be a bad idea
I found this UL announcement;
http://ul.com/newsroom/publicnotices/ul-warns-of-solid-state-relay-with-counterfeit-ul-recognition-mark-release-13pn-52/
The thing to look out for should be that the notching in the lower right corner of the sticker, has a matching protrusion in the case, preventing the sticker to accidentally being applied the wrong way (so the input and output terminals would be swapped).
Also, I noticed that the stickers does seem to have a difference in appearance. However, I don't know how reliable that is.
For example the text on the genuine:
One of the reasons I don't know how reliable that is, is that this picture is found on Fotek's own product page:
Which itself looks a bit different, but might be due to them not updating their site (sorry for the small size, it seems to be what is available from Fotek). Notice though that on neither the image from UL or from Fotek, the text "Made in Taiwan" appears, (it's either "Taiwan made" or nothing).
I've done some light searching on the usual suspects, to see if I could find
something, that
could be genuine and found this:
http://www.ebay.com/itm/Fotek-Solid-State-Relay-SSR-40DA-SSR40DA-New-In-Box-QTY-5-Per-Lot-/291454258612
Now, I know. You can't trust the images on eBay to be representative of the actual product, but at least it has the actual sellers water mark and the image does seem to show a genuine SSR as per the UL announcement.
There might be cheaper ones than this, but at $173 for 5 pieces, (that is $34,60 per SSR), even if a cheaper source of genuine SSR's is to be found, they probably won't come at $6 a pop, like
some claim (you know who I'm talking about...).
Now, this does not mean that there could be better knockoffs to be had. Or that the Mager ones could be "genuine" (or at least of better quality than most Fotek knockoffs).
Edit: I still maintain though, that ~$4 for one of the Fotek knockoffs, is not a bad deal for what you get. You just shouldn't put a lot of load on them. They can have a use, but just use them with care and for low power applications.
Edit2:
This instructable also has some good info on the subject. Someone mentioned in the comments that PQLYT SSR's seems to be performing quite well also.
Here's a link to one of those. It seems to be a few bucks more expensive, but they state it uses a BTA41 800B triac and it also seems to be sealed properly as well. Might not be a bad deal.