thanks to all for the feedback - Much appreciated!
Anyone know the best way to test the spectrum?
If you aren't prepared with a reliable way of loading a program via a known-working and configured properly tape deck - or something like an mp3 player or your phone if you know those work - then as you've guessed you might have a bit of a sod of a time actually testing it out.
Either find a Spectrum nerd that can do it for you; send it to someone like
these guys who will also do stuff like replacing capacitors (you will likely be able to get the money back on sale as a 'fully serviced' example of a toast rack); sell it untested; or roll your sleeves up and dive right in yourself.
Getting loaded tapes to work is an art in itself, so your first steps would be to find a TV with an RF connector still... and tune that sucker in. Hopefully you'll see a working screen and will be able to test some BASIC programs at least.
In all honesty these things aren't the most trivial to work on, and 99% of the time, the keyboard membrane rubber will have perished and the keyboard will be utterly shot - and even if it works, it will not survive the case being opened and it will need replaced on opening for service. I wouldn't take on the hassle of this if you just wanted to sell it on. But if you have an interest... then this fellow called Brendan Alford has written an absolutely exhaustive guide to repairing ZX Spectrums of many makes and models and I consider him the authority for repairs on these shores. It's heavy going and waaay too much detail in some places - but it will answer nigh-on everything, including all that might go wrong and how to repair them.
https://spectrumforeveryone.com/wp-content/uploads/2017/08/ZX-Spectrum-Service-Manual.pdf
Actually, in fact that's an idea... he runs a Facebook group and if you join it, you'll be able to get advice and possibly free help checking it out from a local Speccy nutter. Almost certainly your best bet.
https://www.facebook.com/groups/spectrumforeveryone/
Just be warned that they're not going to be the kinds of people that you'll sell it on for 200+... they're like us and they are used to hunting and buying toastracks for 80 quid or so... which is still a fair amount higher than 'free out of a bin' or '10 quid for a rubber key model in a charity shop window' or 'free from another group member because they couldn't be ****d with having an eighth copy of this model' that they usually brag about on there