[Boromir88]

So did Saruman and Denethor even while they used the Palantiri. They had succumbed, but they were able to maintain the appearance of normalcy.~mark

The only physical signs Denethor had shown was aging, he looked older than he actually was:

'In this way Denethor gained his great knowledge of things that passed into his realm, and far beyond his borders, at which men marvelled; but he bought the knowledge dearly, being aged before his time by his contest with the will of Sauron.~Appendix A: The Stewards

And Saruman was already in an old man's body, so I doubt he was physically different.

As far as any sort of other signs of abnormalcy, I can't think of anything anyone would have noticed. Saruman was able to lull and trick Radagast into fetching Gandalf, so he could put on a fair cloak and act 'normal' for Saruman.

And Denethor, no one seemed to know about his use of the palantir. While he went drastically down hill, that seemed to me to be more out of grief and despair. The palantir heightened and compounded his despair becuase of what Sauron had shown him, but Faramir's apparent death was what sent Denethor over the edge:

Denethor remained steadfast in his rejection Sauron, but was made to believe that his victory was inevitable, and so fell into despair. The reasons for this difference were doubt that in the first place Denethor was a man of great strength of will, and maintained the integrity of his personality until the final blow of the (apparently) mortal wound of his only surviving son.~Unfinished Tales

It is noted also in UT that the palantir was a mental strain on Denethor, and contributed to his grimness, which hastened Finduilas' death:

'The use of the palantiri was a mental strain, especially on men of later yeras not trained to the task, and no doubt in addition to his anxieties this strain contributed to Denethor's 'grimness'. It was probably felt earlier tby his wife than by others and increased her unhappiness, to the hastening of her death.'

So, as far as 'mental' signs Denethor became more grim (but that means he was already a 'grim' man before he looked into the palantir). He still was able to appear and look normal up until he believed Faramir had died, then he just lost his mind.