These are an easy win if there are any present. A particular Horizon Photo Centre wallet included a leaflet containing vouchers, which expired between the end of September and November 1990. This puts an upper limit on the year of the set of around early September (assuming that that vouchers would cease to be included some time before they were due to expire, giving recipients a chance to do something useful with them). It's likely that such a promotion would have run for several months, or even up to a year
The design on the wallets can sometimes help, as most companies revise their designs over time. I had noticed that Horizon (which I used for a few years in the early 90s) changed their design between around 1990 and 1991. Once I'd identified any one set in a particular "design group", I could be fairly confident that the others were of the same vintage.
A York Photo wallet contained an advert for enlargements printed on the inside. Also, in small print was the phrase "Prices valid until 31st May 1998" giving a useful clue, although note that this sort of promotion would probably have been valid for several months.
Sometimes t-shirts can give some useful information. Obviously, those featuring the face of Che Guevara have been around for ever, however on a set of photos of a party in Wales, one of the party-goers can be seen wearing a band t-shirt. In this case, it's for the album Flood by They Might Be Giants. A quick look on Wikipedia reveals that this album was released in 1990. As most people would be unlikely to get access to this sort of merchandise more than a few weeks before a release, it gives a reasonably reliable minimum date for the photo as no earlier than January 1990. However, the fact that it is clearly summer and the photo wallet also contained promotions that expired towards the end of 1990 mean that the set must have been from around July or August 1990.
Many items of clothing can give an indication of at least an approximate era - it's generally easy to spot the difference between the 1970s, 1980s and so on, and some fads (like deelie-boppers in 1982) are short-lived enough to be useful markers.
A single car in a photo will only give a minimum date, and that's not a very reliable one as it could be an old banger or a cherished classic, either of which could be ten or twenty years old. It's also legal to put an older registration on a vehicle, or to transfer original plates without a year. However, a photo which includes a group of cars (especially a car park) should give a good hint as to how old a photograph is. Obviously, the most recent number plate gives a maximum age (i.e. in the UK an 02 or 52 plate means the photo can be no older than 2002), but if there are several of the same year, then it's more likely that the cars are already a year or two old (there are never that many brand new cars around, unless it happens to be a photo of a garage forecourt). Other forms of transport are less reliable, but can give a guide if they look particularly new (although coaches are a problem as coach companies seem to like using original-issue number plates to disguise the age of their vehicles). Trains can also set a minimum age (as long as it's a mainline service, and not some heritage line), however some - like a Class 90 loco or generic Mark 3 coaches, have been around for 20 years or more. If you're really nerdy, however, you can research the history of the loco (especially if it's a separate engine, as trainspotters tend to document all of this on Wikipedia) and combine that with the current operator's livery on the carriages. Since privatisation, these operators can change every few years, and so may narrow down a particular year.
If you're really lucky, you might spot a newspaper, calendar or a gig poster on a wall, or something else similar with a short or measured shelf-life. In the absence of CSI-like infinite-zoom-and-sharpen tools, get the pixel-peeper tool (the magnifier) out in your favourite image editor and scope around any likely photos (you'll probably need a decent 2000dpi, or up, film scan, unless the poster or newspaper is close up, in which case you've probably seen it anyway). Gig posters are likely to be up for several weeks before the even, and may hang around for a short while afterwards. A calendar, as well as showing the year, is likely to have the current month displayed and may even have had the previous days crossed off (note that this doesn't work too well for derelict buildings and offices, where the thing may have been stuck to the wall for a hundred years)
Remember that if you see a day of the week with a particular date, then you can consult old calendars for the year in which the day matches the date - this can narrow things down to plus or minus a few years. For instance, Thursday June 9th occured in 1983 and not again until 1988.
With the exception of events like the Olympics or the Jubilee, you might not directly get a year from a particular event, but you can often get the month or an approximate date given that many events recur at about the same time each year (furthermore, many events will publish some sort of history on their website). Sometimes, one-off events can be easily dated. For instance a set of negatives which had already been dated to sometime in 1990 contained some photos of a flypast in London. A quick internet search reveals that this must have been the 50th Anniversary of the Battle of Britain flypast, held on 15th September.
For most amateurs, a particular set of photos rarely finishes exactly at the end of a roll or starts at the beginning. So don't forget to remember how the beginning and ends of rolls of film can match up to provide, if nothing else, a continuous timeline. Dating any one group of photos can help to date the rest. Bear in mind that most things probably happen at the weekends, so subsequent or previous events are likely to be multiples of seven days. Your general shooting rate might give an indication of roughly how long intervals might be.