I have a losi 8 2.0 and my radio is telling me I am getting too hot at 210 degrees. So my question is how hot is too hot for a nitro engine

I would say around 240 but its different with each engine, tune, clutch setup, ect. 250 is my max.. but thats me..... I also dont trust telemetry systems.. IMO

You can adjust the temp settings on your radio, if its a spectrum that is. I don't know about others. Like previously stated, telemetry is usually suspect....

How accurate is the telemetry in the spectrum cause that’s what I have? And is there a way I can test the heat. The manual says drop water in the head and see how fast it disappears.

Yep, the old spit test on the head is ball park accurate enough for most situations.
Put a glob of spit on you finger tip and drop it on the part of the cooling head that turns down and inwards towards the glow plug.
Rule of thumb is -

If the spit just sits there and does nothing - it's too cold.

If the spits sizzles and disappears within a couple of seconds - it's too hot

If the spit bubbles a bit and then boils / evaporates away over a period of 6 - 7 seconds - it's pretty good (probably around that 190 to 210 range where most engines seem pretty happy.

As mentioned there are a lot of variables as to how hot each individual engine will like to run on a good tune.
If your getting that 6 - 7 sec spit test and the engines performing well and blowing nice smoke - then it's a happy engine. Don't worry about temps any more than that.

210 is very cold imo. Most engines run around 260, Ive had an engine that ran 300 degrees and it didnt have any kind of damage or shortened life span. If it is tuned correctly it probably wont overheat. Never tune with temperatures either

Dont forget running a cold engine is gonna wear the sleeve out sooner also

Yea it all depends on the engine to. Some run cold some run hotter. I think I remember lutz saying he had engine that ran 250 and another 290 with same settings. Main thing is the tune and how it feels. A lot of people say alpha engines are better being really hot.
But the main thing is if you have proper tune and smoke coming out.

Ive run engines at and over 300f for years.

Tuning is based on performance and sound. Temperature and smoke mean next to nothing.
On a windy day, you'll have lower temps. And some fuels smoke more than others.

I'm sure I'll catch flack for my opinion but I've yet to cook an engine, ever.
So many newbies get caught up in tuning to temps that it makes nitro seem so difficult. If it's running good, just keep running it. If it's too lean it WILL sound different and performance will suffer.

i dont even pay attention to heat anymore unless i'm breaking something in. base yourself off of sound and performance.

Yup, break in Is the main time you should be worried about heat and also if your going to drive in cold then preheat of engine you can monitor temp. But everything else is tune.

Alphas like to run hot 250 to 300 is common

This right here.

Tune for performance by sight and sound and then check temps. Im averaging about 240-260F and one hell of a race tune