How to tell if an old SLR camera works?

I'm familiar with DSLR camera (I have a nikon D3000). I want to get into film, and I always see old slr film cameras at the flea market. Before I buy one, how can I tell if the camera works? Do I carry around my own batteries and film to test them out? I know my DSLR can't be exposed to light/dust when I take off the lens, so I'm guessing it's the same with an SLR. That's all I really know.

While dust should be avoided in all cameras, it isn't as much of an issue in SLRs - because it gets a new frame of film with every shot. In DSLRs the sensor stays in place and might collect dust over time.

Bringing film won't do anything to help you to determine if a SLR works - you'd have to develop it to see if e.g. The focusing screen is properly aligned and if the light meter worked well (selenium light meters become inaccurate with age). You could bring batteries and at least try if the shutter works - for that you don't need film.

Carrying around your own batteries won't help - they all use different!

I just bought a used EOS DSLR body off Ebay for £10. The seller told me it worked and he had 100% feedback so I took a risk. Had to buy the battery on-line - couldn't find one locally. Seems to work - everything seems to do what it should and I'm running a film through it for the final check.

I guess what I'm saying is that for £10 + £3 for the battery, £2.50 for the film and whatever to get it processed I'm prepared to take the gamble!

Thanks for your Q but I'm not enough for your answer. Please Don't mind I'm your Friend

Test it at all settings checking for shutter sound and aperture blade response. Don't forget to look through the viewfinder. You may not like what you see and if you do, refuse to buy.

Also remember to open the back where the film goes and check the foam seal, my girlfriends old canon ae-1 had to have the foam replaced. Everything else can be fine but deteriorated foam can cause light leaks and ruin a whole roll of film and every one you run through till you replace it. Replacing it only takes 10 minutes and you can go to a craft store and buy foam and cut it or order kits online

Ignore shutter sound. Whoever suggested that does not realise that many SLR shutters make the right noises but are not opening.
Sme SLRs have no meter. Some have an external Selenium meter - like Zenit E or EM, Praktica LB etc. Some have an internal meter needing a battery BUT the shutters are completely mechanical: Pentax Spotmatic, most Prakticas (MTL, LTL, xLC) and the heavy Chinon CS/CX/AM-1.
Some have electronic shutters needing battery power. I would avoid these because they are hard to test.
Carry an LR9 and an LR44 battery - these have different serial number depending on maker but one of each will allow you to test the meter on many makes of camera.
If the shutter is mechanical, set it to the longer settings, 1, 1/2, 1/4 etc and see if they appear close to accurate.
With the back open, check the wind-on. The sprocketed drive shaft should move exactly 8 teeth per wind on.
Have a look at the rewind spindle - a little pop out handle sometimes breaks free and makes rewind difficult by it's absence.
Take the lense off and have a good look through it - against an angled bright light if possible. This will show surface marks and internal dust but you need to look for fungus. Most screw fitr lenses have an auto pin on the back. Set the lense aperture to f11 and make sure that depressing that pin causes the lense to stop down. Bayonet fittings like Pentax K, Nikon, Canon, etc have a lever that needs activating to open the iris when the lense is off the camera. While on the camera, setting to f22 and firing the shutter should allow you to see the iris stop down when the shutter is fired - look from the front of course.
Good luck! There are some lovely quality screw lenses out there that easily convert to Canon dSLRs. Any Nikon lenses you find will fit your D3000.