ZX Spectrum 16k, ZX Spectrum 48k and ZX Spectrum+ machines all have an internal "loud" speaker (although not that loud, especially with early issue machines ...), but none of these beep when powered up.
So you don't need to connect any display to see if they are working. Power on and press and hold a letter or number key, and if it is working, you should hear the key "click" noise.
Reset (power down/up with the rubber key machines) and blind "type" BEEP 5,5
That is press [EXTEND MODE] (a key on the plus, [CAPS SHIFT] and [SYMBOL SHIFT] on the rubber key), then press and hold [SYMBOL SHIFT] while pressing [Z], then enter [,][ENTER].
Note on the rubber key, you get a comma [,] by pressing [SYMBOL SHIFT] and [N].
You should then hear a tone from the speaker.
Of course, if the keyboard membrane has failed, part or all of the keyboard won't work, and membrane failure is rather common with machines that have been used a lot.
If you have a TV with a composite video input (normally the physical connector is a phono / RCA / Cinch connector with yellow insulation or a SCART connector) then a ZX Spectrum 16k, ZX Spectrum 48k or ZX Spectrum+ machine can be modified fairly easily to output composite video instead of UHF.