Question about determining temperature of fermentation

All I have is one of those glass dairy thermometers. If I put a cup of water right next to the fermenter, can I get an accurate reading off of the water? Or do I need to add a few degrees due to fermentation activity?

During ACTIVE fermentation your temp is going to be several Deg higher then ambient temps. The best way is to have a probe directly in the chamber. If that’s not possible, a stick on is a great way to tell and get close.

Oh ya.... Welcome to the forum!

+1 for the stick on. They are called fermometers.

16" to 18" Red Alcohol Lab thermometers are excellent for this and most even come withe a little rubber anti-roll thingamabob that works great to keep the therm suspended. I find mine fits perfectly through the grommet bung on my buckets and even through the hole in a stopper and still achieves the min. req'd submersion.

I consistenly hit 10*F above ambient during high kreusen.

I tape the temp probe to the side of the carboy then cover that with a couple layers of bubble wrap (just tape it on top). That way the temp probe can get pretty close to what the fermentation temp is, I don't care what the ambient temp is... unless it is cold enough to freeze my airlock

How accurate will reading the water next to the fermenter be though?

I have a floating Thermometer, I leave it in the plastic tube, just float it in there with sanitizer and then strap it to the Primary with the brew belt.

That way I know how much heat the brew belt is providing aprox what the wort temp is in the fermenter. It's still a few deg low most likely, but as I don't let it get over 70. I'm in the ball park.
I record the temps twice a day (morning and night) to watch the fermentation accelerate then drop off. I then know when to plug in the brew belt to keep temps up to continue fermentation and avoid the stalling of the yeasties.
Then when it falls to 68 deg F with the brew belt on, I turn it off and record the drop to room temps (total time 8-10 days) Then I'm ready to rack to 2ndary and record the SG.

This way I'm not opening the Primary to get samples for SG testing. The yeast knows when it's done. The SG is just to get the result and figure ABV.

Works for me, but there are always other ways of doing things. Beer brewing is an individual process thing, and we all have our own quirks and it still makes darn good beer!

Cheers