first question i always ask when it involves demons, why do you want to summon a demon? while demon's aren't all evil, they are powerful and for those who are in experienced dangerous.
demons typically smell, there should be a stange lingering odour in the air. it's different for each, and there are a couple that don't produce a smell but are classified under demons. spirits and demons both produce sounds, temperature changes, missing/moved objects, sometimes injuries [though demons the ijuries are deeper] and uneasy feelings. however spirits wouldn't produce odour typically.
never tried to summon a demon before [or anything really] but it should come down to the specific being, i know if a fire dragon is summoned the room grows hotter, and an air dragon the room is far more humid. unless you have a strong sense, you probably won't see the demon, but you'll certainly be able to tell it's their. while i still say you shouldn't, be sure you guard and cast the cirlce well.