I've noticed that crossword setters love the compact 'TAM' but also the theatrical 'TAM O' SHANTER', so the letter-pattern possibilities are pretty varied.
If the grid shows three letters, it's almost certainly A M (TAM). If the enumeration is something like (3,1,7) or an 11-letter slot, think TAMOSHANTER or the hyphenated form 'TAM-O'-SHANTER' — letters usually count without punctuation, so TAM O' SHANTER becomes TAMOSHANTER in a straight fill. Other full-cap types to keep in mind are GLENGARRY (9) and BALMORAL (8). For two or four letters you might see TAMS (4) or just TAM (3) depending on pluralization.
When I'm stuck I scan crossing letters for the distinctive patterns: GLENGARRY or A M for the short fill. Also watch for British vs. American clueing — British quicks love 'tam', cryptics might hide 'tam' inside another word or clue with wordplay. Happy to nerd out over a particularly tricky setter sometime — I always get a little grin when a tiny TAM finishes a long corner of the puzzle.