The eMate memory card occupies the only spare slot on the logic board, so if you can see an empty slot through the memory cover door on the bottom of the unit, you do not have the upgrade. Simple as that.
One good way of finding these cards, however, is related to this observation: if you are looking on eBay, look for a shot of the bottom of the machine (or ask the seller if it has an empty slot — all you need to tell the seller is to look at the thing in good light); very often the seller has no idea that one of these rare cards is in the machine in question, or what the card is worth. An upgrade card is in fact typically worth more on eBay than the eMate itself, so your goal should be to find an eMate with one of the cards already installed, the value of which the seller does not know.
If there is anyone in the market, there was an eMate memory upgrade card for sale a couple of days ago on eBay. I haven't checked since, but it might be of interest here. For those who actually use the machine, it is a worthwhile upgrade that makes an eMate very usable. The benefit had is not only that it expands the storage of the machine, but also that it widens the data path from (as I recall) 16- to 32-bit.
I have eMates that I installed these cards in and they became noticeably snappier. The eMate is no speed demon at the best of times, of course, but it is fast enough for its intended uses.
Still, it is surprising what these things will do. Someone asked about must-have programs. I think that the various add-ons for Works are must-haves (Calculations, Spreadsheet, Drawing and there was even a low-end Database for it that can be had from unna.org). My own favourite, I think, is a text-to-speech program that outputs rather high quality sound (so long as it is played through something other than the little speaker!). Try it with headphones and you will be in for a real surprise.
I find the text-to-speech function generally to be a useful add-on, as I do a lot of writing and very often will edit my work by having it read to me by a machine, while I sit with pen in hand and made corrections on the fly. Occasionally I will write on an eMate, outdoors, as I have a Keep It Simple Systems solar panel that runs the machine even in partial shade, and that charges the battery at the same time. Amazing.
FilePad is a solid product as a simple database; it even comes with a FileMaker connection utility, and there is a full manual. It is available as freeware. Though it was not the really high-end offering in this sphere, it is still a very decent product on which a simple flat-file database can be constructed (no automated scripts or the like, though, so its uses are limited).
One particular frustration in getting "abandonware" software on the Newton, alas, is that many of the very best of the products available had registration numbers tied to specific machines: you had to send in the machine's details in order to get a key generated by the developer. It was part of the purchase process, which was, interestingly, largely web-based for the Newton even in 1997. The upshot is that, for instance, if you had bought a product in 1997 for your new Newton MessagePad 2000 (like I did), but want to use it today on your recently-acquired-from-eBay eMate 300 in 2010 (like I do) ... you very often can't, or at least you can't without re-registering and paying again. But getting hold of the developers is a real challenge. A good many of the companies simply no longer exist, or have moved on so thoroughly to Palm et al. that there is no longer any sign that they ever had anything to do with the Newton platform. Thus the perfectly sane use of antiquated software for antiquated machines actually can't be always arranged.
Still, there is a good range of freeware available. It is not an ideal situation, but it is good enough to be getting on with. For this, the best source is unna.org.