Research before your call:
As mentioned, do some research on the particular model you're looking at so see if there are common things to look out for or recalls for example (where a manufacturer has found a defect after the car was sold and offered to fix for free). If the seller boasts about having this work done with the paperwork to prove, that's a good sign that they not only knew about it but had it done. Once you've learned enough about the car you're after, you can spot when people are trying it on. See here a dealer advertising a Seat Leon as a K1 (a particular style of body kit) when it's actually not. The white car is an actual K1 with the correct bumper...pretty obvious right?
With only the number plate, you can check a cars MOT history.
https://www.gov.uk/check-mot-history
This will tell you when the car was tested and therefore when the next MOT is due, but also any previous tests and the results. Take note of what, if any, failures there have been but more importantly what advisories have been picked up. As an advisory is not a fail, it's a good sign if these have been repaired as it shows care. If not, bear them in mind when viewing as items that you may need to fix in future.
IF the seller has put pictures of any invoices, I have been known to zoom in and find out where that work was done and call the garage to see if they have any record. Don't expect them to be too helpful as you're not exactly a customer, but it's nice when they are. If nothing else, you can see if the garage is a specialist in those cars or not; if they are, it's a good sign.
If you can't get through to the seller but want a better idea of where the car is, a lot of sites give you a distance based on your postcode. Keep changing your postcode until the distance decreases.
Calling the seller:
As mentioned before, call and ask 'Hi, I'm calling about the car you have for sale', just as a check to see if they're really a private seller or a very good guesser.
Go through and confirm the points in the advert; mileage, length of MOT, service history etc. Ask if the service history is stamps in the book or just receipts for work (or both). Some sellers have a very different opinion on what a 'Full service history' is.
Cam belts Vs Cam chains: Engines can have either a cam belt (rubber) or a cam chain (steel). A general rule is a cam belt needs replacing every so often depending on mileage or age, and a chain is for the life of the vehicle. This isn't a hard and fast rule though so do some research on the car you're looking at and ask the appropriate question. Normally, just asking if the cambelt has been replaced is a good start; if they're keen to tell you it's been done at the correct interval, with the paperwork to prove, it's a good. If they say no because it's a chain, just check.
Other more general questions about length of ownership, how reliable it's been, if there's any damage etc can be taken with a pinch of salt. To be honest, I ask these to gauge the reaction of the seller and see how much information they offer as opposed to what's being asked. Some people aren't necessarily dishonest, but if you ask if there's any rust for example, they simply haven't got down on theirs hands and knees to look, so will quite happily say 'No'.
Negotiating can be hard for some people but the key is for it not to be an awkward topic to broach. Towards the end of the call, ask 'So if it's as described and I'd like to buy it, how negotiable are you on the price? I'm not looking to barter over the phone without seeing it, I'd just like to know how to prepare'.
This, in my view, gently brings up the topic and shows that you are serious but also shows you're not looking to pay full price. You can also use this opportunity to discuss preferred payment; depending on the price of the car, the seller may want a bank transfer or cash. During the viewing, if you do decide to buy, you can reference the call and start from there; 'So you said you were open to sensible offers. How about ......'