The best way to determine temperature stability for your laboratory is to directly measure temperature using a digital recording thermometer. There are many models available and usually cost between $200 to $400. This is the one we use though it is a little more expensive:
http://www.dicksondata.com/products/FH625Because most labs I've worked in have terrible temperature stability control (usually the A/C is insufficient in BTU rating), I implemented a drift correction for the unknown measurements that is automatically applied to all unknowns if they are between standardizations in real time.
Here's an example of several days of operation in our new lab which shows excellent intensity stability:
Drift array standard intensities (cps/30nA) (background corrected):
ELMXRY: na ka si ka k ka al ka mg ka ca ka ti ka mn ka fe ka p ka cr ka
MOTCRY: 1 TAP 2 LPET 2 LPET 4 TAP 1 TAP 5 LIF 3 LLIF 3 LLIF 5 LIF 4 TAP 3 LLIF
STDASS: 336 162 374 336 162 162 22 25 162 285 396
STDVIR: 0 0 0 0 0 0 0 0 0 0 0
2533.6 9875.5 6024.1 8171.4 2855.9 328.8 6415.5 14908.2 596.5 9707.0 5288.6
2522.5 9977.4 6006.2 8341.4 2841.3 338.5 6377.0 14945.2 596.2 9633.9 5166.0
2517.3 9989.2 6010.5 8218.5 2837.0 339.9 6428.2 14926.4 601.5 9695.2 5231.3
2504.6 10013.6 5999.0 8312.4 2850.8 337.9 6456.1 14052.2 606.4 9566.2 5212.1
2522.0 10026.2 5159.5 8303.9 2834.5 337.0 - - 599.4 - -
- 10011.7 - - 2820.3 333.8 - - 600.0 - -
- 9995.9 - - 2855.1 337.5 - - 600.3 - -
But even though the temperature control in our new lab is excellent, we still almost always measure standards before and after all unknown runs because in PFE it is just a couple of mouse clicks (assuming your standards are in the instrument along with your unknowns!), and then you know for certain that the instrument was stable during the entire measurement period.
And if it wasn't, the automatic drift correction in the software takes care of it for you anyway!